Bentley Cameron L, Kang Minkyung, Maddar Faduma M, Li Fengwang, Walker Marc, Zhang Jie, Unwin Patrick R
Department of Chemistry , University of Warwick , Coventry CV4 7AL , UK . Email:
School of Chemistry , Australian Research Council Centre of Excellence for Electromaterials Science , Monash University , Clayton , Vic 3800 , Australia.
Chem Sci. 2017 Sep 1;8(9):6583-6593. doi: 10.1039/c7sc02545a. Epub 2017 Jul 26.
Two dimensional (2D) semiconductor materials, such as molybdenum disulfide (MoS) have attracted considerable interest in a range of chemical and electrochemical applications, for example, as an abundant and low-cost alternative electrocatalyst to platinum for the hydrogen evolution reaction (HER). While it has been proposed that the edge plane of MoS possesses high catalytic activity for the HER relative to the "catalytically inert" basal plane, this conclusion has been drawn mainly from macroscale electrochemical (voltammetric) measurements, which reflect the "average" electrocatalytic behavior of complex electrode ensembles. In this work, we report the first spatially-resolved measurements of HER activity on natural crystals of molybdenite, achieved using voltammetric scanning electrochemical cell microscopy (SECCM), whereby pixel-resolved linear-sweep voltammogram (LSV) measurements have allowed the HER to be visualized at multiple different potentials to construct electrochemical flux movies with nanoscale resolution. Key features of the SECCM technique are that characteristic surface sites can be targeted and analyzed in detail and, further, that the electrocatalyst area is known with good precision (in contrast to many macroscale measurements on supported catalysts). Through correlation of the local voltammetric response with information from scanning electron microscopy (SEM) and atomic force microscopy (AFM) in a , it is demonstrated unequivocally that while the basal plane of bulk MoS (2H crystal phase) possesses significant activity, the HER is greatly facilitated at the edge plane (, surface defects such as steps, edges or crevices). Semi-quantitative treatment of the voltammetric data reveals that the HER at the basal plane of MoS has a Tafel slope and exchange current density () of ∼120 mV per decade and 2.5 × 10 A cm (comparable to polycrystalline Co, Ni, Cu and Au), respectively, while the edge plane has a comparable Tafel slope and a that is estimated to be more than an order-of-magnitude larger (∼1 × 10 A cm). Finally, by tracking the temporal evolution of water contact angle (WCA) after cleavage, it is shown that cathodic polarization has a 'self-cleaning' effect on the surface of MoS, consistent with the time-independent (, time after cleavage) HER voltammetric response.
二维（2D）半导体材料，如二硫化钼（MoS），在一系列化学和电化学应用中引起了广泛关注，例如，作为一种丰富且低成本的替代电催化剂，用于析氢反应（HER）以替代铂。虽然有人提出，相对于“催化惰性”的基面，MoS的边缘平面对于HER具有高催化活性，但这一结论主要是从宏观电化学（伏安法）测量得出的，这些测量反映了复杂电极集合体的“平均”电催化行为。在这项工作中，我们报告了首次使用伏安扫描电化学池显微镜（SECCM）对辉钼矿天然晶体上的HER活性进行空间分辨测量，通过像素分辨线性扫描伏安图（LSV）测量，可以在多个不同电位下可视化HER，以构建具有纳米级分辨率的电化学通量电影。SECCM技术的关键特性在于，可以详细靶向和分析特征表面位点，此外，电催化剂面积的精度很高（与许多负载型催化剂的宏观测量不同）。通过将局部伏安响应与扫描电子显微镜（SEM）和原子力显微镜（AFM）的信息相关联，明确证明了虽然块状MoS（2H晶相）的基面具有显著活性，但HER在边缘平面（如台阶、边缘或裂缝等表面缺陷处）得到极大促进。对伏安数据的半定量处理表明，MoS基面的HER具有约每十倍频程120 mV的塔菲尔斜率和2.5×10 A cm的交换电流密度（与多晶Co、Ni、Cu和Au相当），而边缘平面具有可比塔菲尔斜率且估计交换电流密度超过一个数量级更大（约1×10 A cm）。最后，通过跟踪解理后水接触角（WCA）的时间演变，表明阴极极化对MoS表面具有“自清洁”作用，这与时间无关（解理后的时间）的HER伏安响应一致。
