表面修饰基团在过渡金属氧化物纳米颗粒上成键机理的原位非线性光学研究

Surface modification of functional nanoparticles by attaching ligand molecules to particle surfaces plays crucial roles in regulating the fundamental properties of the nanomaterials, such as the optical and electronic functionalities, the chemical and biological reactivity, as well as the dispersion stabilities in colloidal solutions. However, due to the lack of reliable in situ characterization tools for the buried solid-liquid interface, the nature of the ligand binding structure on nanoparticles in the aqueous environment is still not fully understood. For example, the surface of the transition metal oxide nanoparticles in the aqueous solution is often complicated and under dynamic changes. Up to date, little has been learned about how the fundamental surface properties of the transition metal oxide nanoparticles, including surface potential, oxidation states, particle size, shape, and surface crystal structures, are correlated to the binding structures and attachment affinities of the ligand molecules on nanoparticle surfaces. The second order nonlinear spectroscopy, including the second harmonic generation (SHG) and sum frequency generation (SFG), is known for its intrinsic surface-selectivity and sub-monolayer sensitivity. In addition, as optical spectroscopic probes, SFG and SHG do not require vacuum conditions, making them ideal in situ characterization tools for the study of the molecular structures and reaction dynamics on the nanoparticle surfaces. However, most of the previous SFG studies on nanoparticle surfaces focused on the dry nanoparticle film at the solid-air interface and the detection was often limited in the vibrational range of methyl stretching region, which is characteristic of the end groud of the hydrocarbon chain instead of the anchoring group directly attached to the nanoparticle surface. Here we propose to employ SFG vibrational spectroscopy (SFG-VS) to explore the detailed binding mechanisms of phosphonate, sulfonate, and carboxylate ligand molecules anchored on TiO2, CeO2, and Fe2O3 nanoparticles in the environment of aqueous soultions. We will focus on the infrared region around 10 microns which correspond to the vibrations of the anchoring groups. The dependence of the binding configurations and attachment affinities on the particle size, surface potential, surface oxidation states of the nanoparticles, as well as on the pH and ionic strength of the solutions, will be systematically explored. The anticipated results are going to have great impact on the design and fabrication of the functional nanoparticles, and provide detailed understanding of the dynamics of the nanocatalysis reactions in aqueous soultions.

表面修饰分子对人工调节功能型纳米材料的光电性能及化学与生物反应活性具有决定性的影响。长期以来，由于缺乏针对被埋藏的固-液界面的有效原位测量手段，溶液中纳米颗粒与表面修饰基团的成键机理尚未得到充分认识。过渡金属氧化物纳米颗粒的表面性质，如表面电位、氧化价态、尺寸、形状与晶面等，是如何影响配体分子与纳米颗粒表面之间的成键构型与键能的，一直是悬而未决的重要科学问题。界面非线性光谱具有表面选择性与亚单分子层的灵敏度，是研究纳米颗粒-溶液界面的分子结构与反应动力学的理想工具。本课题拟使用和频振动光谱，并结合纳米材料领域的常规表征手段，系统地研究溶液环境中膦酸、磺酸与羧酸配体分子在TiO2、CeO2以及Fe2O3纳米颗粒表面上的成键机理，深入了解表面分子结构与氧化物纳米颗粒的大小、表面电位、价态等性质之间的依赖关系。本研究对人工调控纳米材料和在分子层次上理解溶液中纳米催化反应动力学机理具有重要意义。

对金属氧化物纳米材料进行表面化学修饰，可以实现对纳米材料的性质与功能的人工调节与控制。但由于长期以来缺乏针对被埋藏的固-液界面的有效原位测量手段，水相溶液环境中的纳米颗粒与表面修饰基团之间的成键机理尚未得到充分认识。在项目中，我们利用具有界面选择性与亚单分子层灵敏度的和频振动光谱，并结合纳米材料领域的常规表征手段，系统地研究了溶液环境中含氧有机酸配体分子在金属氧化物纳米颗粒表面上的成键机理。取得的主要成果包括：（1）研究了气/液界面磷脂与不饱和脂肪酸单分子膜自组装结构，利用偏振依赖的和频振动光谱发现了界面烷烃链分子中甲基反对称伸缩模式的光谱分裂，发展了基于CS分子对称点群模型的长链烃类分子中甲基基团和频振动光谱偏振选择定律，并成功测量了界面磷脂分子中所有官能团的空间取向，为利用和频振动光谱研究界面复杂分子体系奠定了基础。（2）研究了赤铁矿纳米颗粒与气/液界面带有不同电荷的磷脂单分子膜之间的相互作用，发现了赤铁矿纳米颗粒可插入磷脂单分子膜的孔洞之中，并使膜中分子的排列更加致密有序，该部分工作为深入理解赤铁矿纳米颗粒的生物毒性提供了直接的光谱证据。（3）研究了有机小分子在Pd/Al2O3纳米复合结构表面的竞争吸附，发现吸附能力顺序为乙醇~乙酸>乙醛，为进一步研究乙醇在纳米颗粒表面被催化而发生脱氢氧化反应生成乙醛与乙酸的反应机理打下了基础。（4）考察了液体亚相折射率等因素在和频振动光谱定量分析中的影响，发现体相折射率对固/液界面的和频振动光谱强度测量影响较大，并且影响程度与可见光入射角密切相关；同时以空气/液体甲醇界面为例，发现体相折射率对气/液界面的和频振动光谱取向分析的影响也较大，并据此进一步研究了界面空间取向的体相盐离子浓度效应。