Electrochemical properties of CVD grown pristine graphene: monolayer- vs. quasi-graphene
Electrochemical properties of CVD grown pristine graphene: monolayer- vs. quasi-graphene
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DOI：
10.1039/c3nr05643k
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发表时间：
2014-01-01
期刊：
影响因子：
6.7
通讯作者：
Banks, Craig E.
中科院分区：
文献类型：
--
作者：
Brownson, Dale A. C.;Varey, Sarah A.;Banks, Craig E.
我们报告了通过 CVD 生长并使用 PMMA 转移到绝缘基板（二氧化硅晶片）上的原始单层、双层和几层（称为准）石墨烯的电化学特性。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，揭示了前者是“真正的”原始单层石墨烯（O/C 为 0.05），后者是原始准石墨烯（O/C 为 0.07）；由于表面平均包含 4 个石墨烯层，因此创造了术语“准石墨烯”。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的石墨电极（即由高度有序热解石墨（HOPG）构造的基底和边缘平面热解石墨电极的电化学性能）进行比较，并获得有关异质速率常数（ko）的信息。电化学速率常数主要受石墨烯表面电子特性的影响。发现单层石墨烯对所研究的氧化还原探针表现出缓慢的异质电子转移 (HET) 动力学，HET 速率约为 100%。相对于单层石墨烯电极，准石墨烯和 HOPG 分别快 2 倍和 8 倍。严格对比单层石墨烯与准石墨烯和 HOPG 电极的性能表明，增加石墨烯层数会改善电化学性能，其中就所研究的探针的电化学可逆性而言：单层石墨烯 < 准石墨烯 < HOPG，由各自的 HET 电化学速率常数决定。鉴于边缘平面位点是石墨材料快速电子转移动力学的主要来源，原始单层石墨烯电极的慢 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底平面贡献。就准石墨烯和 HOPG 而言，它们分别具有不断增加的电化学反应边缘平面位点的全局覆盖率，因此表现出优于单层石墨烯的电化学性能。最后，考虑双层石墨烯电极的情况，由于其制造，其具有大范围的边缘平面类似位点/缺陷的全局覆盖。与之前的结论一致，双层缺陷石墨烯电极表现出快速/良好的电化学性能，这归因于其较大的边缘平面含量（即缺陷丰富的石墨烯），因此进一步证明电化学响应取决于石墨烯基电极边缘平面位点的密度（受石墨烯缺陷的覆盖范围和石墨烯层数的影响）。
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 (ko) 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 doublelayered 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).