Faculty of Science and Engineering, School of Science and the Environment, Division of Chemistry and Environmental Science, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, Lancs, UK.
Nanoscale. 2014;6(3):1607-21. doi: 10.1039/c3nr05643k.
We report the electrochemical properties of pristine monolayer, double layer and few-layer (termed quasi-) graphene grown via CVD and transferred using PMMA onto an insulating substrate (silicon dioxide wafers). Characterisation has been performed by Raman spectroscopy, optical spectroscopy, Atomic Force Microscopy and X-ray Photoelectron Spectroscopy, revealing 'true' pristine single-layer graphene (O/C of 0.05) at the former and pristine quasi-graphene at the latter (O/C of 0.07); the term "quasi-graphene" is coined due to the surface comprising on average 4-graphene-layers. The graphene electrodes are electrochemically characterised using both inner-sphere and outer-sphere redox probes with electrochemical performances of the graphene electrodes compared to other available graphitic electrodes, namely that of basal- and edge- plane pyrolytic graphite electrodes constructed from Highly Ordered Pyrolytic Graphite (HOPG), with information on heterogeneous rate constants (k(o)) obtained. The electrochemical rate constants are predominantly influenced by the electronic properties of the graphene surfaces. Monolayer graphene is found to exhibit slow heterogeneous electron transfer (HET) kinetics towards the redox probes studied, with HET rates ca. 2 and 8 times faster at quasi-graphene and HOPG respectively, relative to that of the monolayer graphene electrode. Critically contrasting the performance of monolayer graphene to quasi-graphene and HOPG electrodes reveals that increasing the number of graphene layers results in improved electrochemical properties, where in terms of the electrochemical reversibility of the probes studied: monolayer-graphene < quasi-graphene < HOPG, as governed by the respective HET electrochemical rate constants. Given that edge plane sites are the predominant origin of fast electron transfer kinetics at graphitic materials, the slow HET rates at pristine single-layer graphene electrodes are likely due to graphene's fundamental geometry, which comprises a small edge plane and large basal plane contribution. In the case of quasi-graphene and HOPG, they possess increasing global coverage of electrochemically reactive edge plane sites (respectively) and thus exhibit superior electrochemical performances over that of monolayer graphene. Last, the case of a double-layer graphene electrode is considered, which as a result of its fabrication possesses a large global coverage of edge plane like- sites/defects. In agreement with the former conclusions, the double-layered defect-graphene electrode is found to exhibit fast/favourable electrochemical properties, which is attributed to its large edge plane content (i.e. defect abundant graphene) and thus is further evidence that the electrochemical response is dependent on the density of edge plane sites at graphene based electrodes (influenced by the coverage of graphene-defects and the number of graphene layers).
我们报告了通过 CVD 生长并使用 PMMA 转移到绝缘基底（硅晶片）上的原始单层、双层和少层（称为准）石墨烯的电化学性质。通过拉曼光谱、光学光谱、原子力显微镜和 X 射线光电子能谱进行了表征，结果表明前者为“真正”原始单层石墨烯（O/C 为 0.05），后者为原始准石墨烯（O/C 为 0.07）；“准石墨烯”一词是由于表面平均包含 4 层石墨烯。使用内球和外球氧化还原探针对石墨烯电极进行电化学表征，并将石墨烯电极的电化学性能与其他可用石墨电极（即来自高度有序热解石墨（HOPG）的基面和边缘面热解石墨电极）进行比较，获得了关于非均相速率常数（k(o)）的信息。电化学速率常数主要受石墨烯表面的电子性质影响。单层石墨烯对所研究的氧化还原探针表现出缓慢的非均相电子转移（HET）动力学，相对于单层石墨烯电极，准石墨烯和 HOPG 的 HET 速率分别快约 2 倍和 8 倍。与准石墨烯和 HOPG 电极相比，单层石墨烯的性能显著表明，增加石墨烯层的数量会导致电化学性能的改善，就所研究探针的电化学可逆性而言：单层-石墨烯<准-石墨烯<HOPG，这是由相应的 HET 电化学速率常数决定的。鉴于边缘平面位点是石墨材料中快速电子转移动力学的主要起源，原始单层石墨烯电极的缓慢 HET 速率可能是由于石墨烯的基本几何形状，其中包括小的边缘平面和大的基面贡献。对于准石墨烯和 HOPG，它们具有越来越多的电化学活性边缘平面位点的全局覆盖（分别），因此表现出优于单层石墨烯的电化学性能。最后，考虑了双层石墨烯电极的情况，由于其制造，它具有大的全局覆盖的边缘平面样-位点/缺陷。与前一个结论一致，发现双层缺陷-石墨烯电极具有快速/有利的电化学性质，这归因于其大的边缘平面含量（即富含缺陷的石墨烯），这进一步证明了电化学响应取决于基于石墨烯的电极上边缘平面位点的密度（受石墨烯缺陷的覆盖和石墨烯层数的影响）。
