Department of Chemistry , Northwestern University , Evanston , Illinois 60208 , United States.
Nano Lett. 2019 Mar 13;19(3):2106-2113. doi: 10.1021/acs.nanolett.9b00313. Epub 2019 Feb 19.
Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was used for the first time to spatially resolve local heterogeneity in redox behavior on an electrode surface in situ and at the nanoscale. A structurally well-defined Au(111) nanoplate located on a polycrystalline ITO substrate was studied to examine nanoscale redox contrast across the two electrode materials. By monitoring the TERS intensity of adsorbed Nile Blue (NB) molecules on the electrode surface, TERS maps were acquired with different applied potentials. The EC-TERS maps showed a spatial contrast in TERS intensity between Au and ITO. TERS line scans near the edge of a 20 nm-thick Au nanoplate demonstrated a spatial resolution of 81 nm under an applied potential of -0.1 V vs Ag/AgCl. The intensities from the TERS maps at various applied potentials followed Nernstian behavior, and a formal potential ( E) map was constructed by fitting the TERS intensity at each pixel to the Nernst equation. Clear nanoscale spatial contrast between the Au and ITO regions was observed in the E map. In addition, statistical analysis of the E map identified a statistically significant 4 mV difference in E on Au vs ITO. Electrochemical heterogeneity was also evident in the E distribution, as a bimodal distribution was observed in E on polycrystalline ITO, but not on gold. A direct comparison between an AFM friction image and the E map resolved the electrochemical behavior of individual ITO grains with a spatial resolution of ∼40 nm. The variation in E was attributed to different local surface charges on the ITO grains. Such site-specific electrochemical information with nanoscale spatial and few mV voltage resolutions is not available using ensemble spectroelectrochemical methods. We expect that in situ redox mapping at the nanoscale using EC-AFM-TERS will have a crucial impact on understanding the role of nanoscale surface features in applications such as electrocatalysis.
电化学原子力显微镜针尖增强拉曼光谱（EC-AFM-TERS）首次被用于原位和纳米尺度上空间分辨电极表面氧化还原行为的局部不均匀性。在多晶 ITO 衬底上研究了结构明确的 Au(111)纳米板，以检查横跨两种电极材料的纳米尺度氧化还原对比。通过监测电极表面吸附的尼罗蓝（NB）分子的 TERS 强度，在不同施加电位下获得 TERS 图谱。EC-TERS 图谱显示 Au 和 ITO 之间 TERS 强度存在空间对比度。在施加电位为-0.1 V 相对于 Ag/AgCl 时，在 20nm 厚的 Au 纳米板边缘附近进行的 TERS 线扫描显示出 81nm 的空间分辨率。在各种施加电位下的 TERS 图谱的强度遵循 Nernst 行为，并且通过将每个像素的 TERS 强度拟合到 Nernst 方程来构建 E 图谱。在 E 图谱中观察到 Au 和 ITO 区域之间明显的纳米尺度空间对比度。此外，E 图谱的统计分析确定了 Au 与 ITO 之间 E 值的统计学上显著的 4mV 差异。在 E 分布中也可以明显看出电化学不均匀性，因为在多晶 ITO 上观察到 E 的双峰分布，但在金上则没有。AFM 摩擦力图像与 E 图谱的直接比较解决了具有约 40nm 空间分辨率的单个 ITO 晶粒的电化学行为。E 的变化归因于 ITO 晶粒上不同的局部表面电荷。使用集总光谱电化学方法无法获得具有纳米级空间分辨率和几毫伏电压分辨率的这种特定于位置的电化学信息。我们预计，使用 EC-AFM-TERS 进行纳米尺度的原位氧化还原映射将对理解纳米尺度表面特征在电催化等应用中的作用产生关键影响。
