Institute of Chemistry and Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
J Chem Phys. 2021 Nov 28;155(20):204704. doi: 10.1063/5.0072079.
The performance of catalysts depends on their nanoscale properties, and local variations in structure and composition can have a dramatic impact on the catalytic reactivity. Therefore, probing the localized reactivity of catalytic surfaces using high spatial resolution vibrational spectroscopy, such as infrared (IR) nanospectroscopy and tip-enhanced Raman spectroscopy, is essential for mapping their reactivity pattern. Two fundamentally different scanning probe IR nanospectroscopy techniques, namely, scattering-type scanning near-field optical microscopy (s-SNOM) and atomic force microscopy-infrared spectroscopy (AFM-IR), provide the capabilities for mapping the reactivity pattern of catalytic surfaces with a spatial resolution of ∼20 nm. Herein, we compare these two techniques with regard to their applicability for probing the vibrational signature of reactive molecules on catalytic nanoparticles. For this purpose, we use chemically addressable self-assembled molecules on Au nanoparticles as model systems. We identified significant spectral differences depending on the measurement technique, which originate from the fundamentally different working principles of the applied methods. While AFM-IR spectra provided information from all the molecules that were positioned underneath the tip, the s-SNOM spectra were more orientation-sensitive. Due to its field-enhancement factor, the s-SNOM spectra showed higher vibrational signals for dipoles that were perpendicularly oriented to the surface. The s-SNOM sensitivity to the molecular orientation influenced the amplitude, position, and signal-to-noise ratio of the collected spectra. Ensemble-based IR measurements verified that differences in the localized IR spectra stem from the enhanced sensitivity of s-SNOM measurements to the adsorption geometry of the probed molecules.
催化剂的性能取决于其纳米级特性，结构和组成的局部变化会对催化活性产生巨大影响。因此，使用高空间分辨率振动光谱（如红外（IR）纳米光谱和针尖增强拉曼光谱）探测催化表面的局部反应性对于绘制其反应性模式至关重要。两种基本不同的扫描探针红外纳米光谱技术，即散射型近场光学显微镜（s-SNOM）和原子力显微镜-红外光谱（AFM-IR），提供了以约 20nm 的空间分辨率绘制催化表面反应性模式的能力。在这里，我们比较了这两种技术在探测催化纳米粒子上反应性分子的振动特征方面的适用性。为此，我们使用化学寻址的自组装分子在 Au 纳米粒子上作为模型系统。我们根据测量技术确定了明显的光谱差异，这些差异源于应用方法的基本不同工作原理。虽然 AFM-IR 光谱提供了位于探针下方的所有分子的信息，但 s-SNOM 光谱对取向更敏感。由于其场增强因子，s-SNOM 光谱显示出与表面垂直取向的偶极子更高的振动信号。s-SNOM 对分子取向的敏感性影响了收集光谱的幅度、位置和信噪比。基于集合的 IR 测量验证了局部 IR 光谱的差异源于 s-SNOM 测量对被探测分子吸附几何的增强敏感性。
