催化学报 ›› 2019, Vol. 40 ›› Issue (11): 1655-1667.DOI: 10.1016/S1872-2067(19)63357-7

• 综述 • 上一篇    下一篇

基于偏振依赖和频振动光谱模拟分析界面分子取向

Xia Li, Günther Rupprechter   

  1. 维也纳技术大学材料化学研究所, 1060 维也纳, 奥地利
  • 收稿日期:2019-01-29 修回日期:2019-03-18 出版日期:2019-11-18 发布日期:2019-09-06
  • 通讯作者: Günther Rupprechter
  • 基金资助:
    This work was in part supported by the Austrian Science Fund FWF through projects ComCat (I 1041-N28) and DK+ Solids4Fun (W1243) and by TU Wien via the Innovative Project "SFG Spectroscopy". X. Li acknowledges the doctoral candidate Jian Hou at Institute of Chemistry, Chinese Academy of Sciences for providing the picture of experimental setup in Fig. 2c.

A modeling analysis of molecular orientation at interfaces by polarization-dependent sum frequency generation vibrational spectroscopy

Xia Li, Günther Rupprechter   

  1. Institute of Materials Chemistry, Technische Universität Wien, 1060 Vienna, Austria
  • Received:2019-01-29 Revised:2019-03-18 Online:2019-11-18 Published:2019-09-06
  • Contact: Günther Rupprechter
  • Supported by:
    This work was in part supported by the Austrian Science Fund FWF through projects ComCat (I 1041-N28) and DK+ Solids4Fun (W1243) and by TU Wien via the Innovative Project "SFG Spectroscopy". X. Li acknowledges the doctoral candidate Jian Hou at Institute of Chemistry, Chinese Academy of Sciences for providing the picture of experimental setup in Fig. 2c.

摘要: 和频(SFG)振动光谱由于具有很强的偏振依赖性, 已被证明是测量表面/界面分子结构、对称性和取向的一种很好的工具. 然而, 对界面上的SFG光谱强度和分子取向的精确定量分析必须仔细地进行. 本本文总结了在分析SFG光谱中常被忽视的参数和因素, 并通过考察在气体(超高真空, UHV)/固体界面处CO在Pt和Pd单晶(111)面的吸附以及在氯(碘)化钠盐存在下甲醇(水)在气(固)/液界面上的吸附加以说明其影响. 为了直观地估计激光入射角和介质折射率对SFG强度的影响, 只讨论了一个定义的因子|Fyyz|, 它可以从宏观二阶非线性磁化率项χyyz(2)中单独分离出来, 并可代表SSP强度. 此外, 在界面CO和甲醇分子取向分析中还讨论了折射率和分子超极化率(R)的影响. 当IPPP/ISSP相同时, R值较大的分子在Pt上的倾斜角(θ)较小(假设θ<51°), Pd上吸附的CO分子比在Pt上更靠近表面. 全内反射(TIR)几何构型增强了SFG的强度, 但也放大了折射率对固(SiO2)/液界面上SFG强度的影响. 在碘化钠作用下, 折射率和R值的变化对甲醇分子在气/液和固/液界面取向的影响相似. 本文可为分析具有不同R值的分子在涉及折射率变化的催化剂和液体界面处的取向提供指导.

关键词: 和频光谱, 表面/界面, 多相催化, 分子取向, 折射率, 入射角, 分子超极化率

Abstract: Sum frequency generation (SFG) vibrational spectroscopy has been proven an excellent tool to measure the molecular structures, symmetries and orientations at surfaces/interfaces because of its strong polarization dependence. However, a precise quantitative analysis of SFG spectral intensity and molecular orientation at interfaces must be carefully performed. In this work, we summarized the parameters and factors that are often ignored and illustrated them by evaluating studies of CO adsorption on the (111) facet of platinum (Pt) and palladium (Pd) single crystals at the gas (ultra-high vacuum, UHV)/solid interfaces and methanol (water) adsorption at the air/liquid (solid/liquid) interfaces in the presence of sodium iodide (chloride) salts. To intuitively estimate the influence of incidence angles and refractive indices on the SFG intensity, solely a defined factor of|Fyyz|was discussed, which can be individually separated from the macroscopic second-order non-linear susceptibility χyyz(2) term and represents the SSP intensity. Moreover, effects of refractive indices and the molecular hyperpolarizability ratio (R) were discussed in the orientational analysis of interfacial CO and methanol molecules. When IPPP/ISSP was identical, molecules with a larger R had smaller tilting angles (θ) on Pt (assuming θ<51°), and CO molecules on Pd would tilt much closer to the surface than they did on Pt. A total internal reflection (TIR) geometry enhanced the SFG intensity, but it also amplified the influence of refractive index on SFG intensity at the solid (silica)/liquid interface. The refractive index and R-value had similar influence on the methanol orientation in the presence of sodium iodide salts at air/liquid and solid/liquid interfaces. This work should provide a guideline for analyzing the orientation of molecules with different R, which are adsorbed on catalysts or located at liquid interfaces involving changes of refractive indices.

Key words: Sum frequency generation, spectroscopy, Surface/interface, Heterogeneous catalysis, Molecular orientation, Refractive index, Incidence angles, Molecular hyperpolarizability ratio