甲醇在TiO2(110)表面上微观结构处的光化学研究

Methanol can greatly enhance the efficiency of water splitting over TiO2, thus the chemical properties of methanol on TiO2 has attracted much attention. However the micro interaction mechanism and dynamical process of methanol on TiO2 are still not clearly understood, so it is necessary to study the photochemical processes in molecular level. There are several types of microstructures on TiO2(110) surface, such as terraces, steps as well as distinct micro zones, which are formed during the processing of sample and seem different from the surrounding surface areas. It is possible for methanol to show diverse adsorbing states and photochemistry on these microstructures and thus a micro-zone analysis of them become necessary. Two-photon photoemission (2PPE) spectroscopy is an excellent method to detect the electronic structure and excited state dynamics in surface reactions. However, limited by traditional data collecting method it can only obtain spectra from large surface area on average. In this project, we plan to combine 2PPE with SPELEEM (Spectroscopic photoemission and low energy electron microscopy) together, which will enable the collection of spectra from selected micro zones (in several hundred nanometers) on sample surface. We are going to acquire the 2PPE spectra of methanol adsorbed on different surface microstructures so as to study the adsorbing states and photochemistry of them. It is hopeful that this project will promote the understanding of the interactions of methanol with TiO2, and benefit the searching for catalysts with high efficiency in hydrogen production.

甲醇能提高TiO2光催化分解水产氢的效率，因此甲醇在TiO2上的光化学性质备受关注。但是人们对于甲醇与TiO2相互作用的微观机理和动力学过程依然不清楚，所以从分子水平上研究甲醇在TiO2上的光化学过程是很有必要的。TiO2(110)表面上存在着台面、台阶以及随样品处理而出现的与周围环境不同的局部区域或微观结构。甲醇在不同结构处可能会表现出不同的吸附状态和光化学性质，因而有必要从微区上对它们分别加以分析。2PPE能谱非常适合用来探测表面化学反应中的电子结构和激发态动力学，但传统的光电子能谱采集手段只能获得大面积表面的平均效果。本项目拟将SPELEEM和2PPE技术相结合，利用SPELEEM的微区(数百纳米)能谱功能，分别探测甲醇在表面各种微观结构处的2PPE能谱，研究其吸附状态和光化学性质，加深对甲醇与TiO2(110)表面相互作用的理解，为发展高效产氢催化剂提供线索。

本项目着眼于利用SPELEEM结合飞秒激光研究甲醇在TiO2(110)表面上的特征微观结构处的光化学性质，以促进对于甲醇与TiO2相互作用的微观机理和动力学过程的理解。在项目执行过程中，我们研究了TiO2(110)表面的调控制备，能够获得满足实验要求的表面微结构。氧气氛下退火对表面具有强烈的塑造作用，氧压和退火温度共同决定了表面的生长方式，低退火温高氧压有利于表面上两种再构以更快的周期交替生长并得到更光滑的表面，反之则生长速度慢，表面粗糙。低温下TiO2(110)-1X1表面沉积甲醇后的2PPE能谱测量显示，随甲醇沉积量增多，由甲醇在Ti5c处解离产生的中间态峰逐渐出现、增强并向高能移动，光照会使得其强度迅速增加并达到饱和。此外利用X射线观察了表面上两种重构的微区电子能谱及电子结构特征的空间分布，完成了飞秒激光pump-probe光路的建设，使PEEM具备了研究超快动力学过程的时间分辨能力。