The Crystalline Nanocluster Phase as a Medium for Structural and Spectroscopic Studies of Light Absorption of Photosensitizer Dyes on Semiconductor Surfaces

The crystalline nanocluster phase, in which nanoscale metal oxide clusters are self-assembled in three-dimensional periodic arrays, is described. The crystalline assembly of nanoparticles functionalized with technologically relevant ligands offers the opportunity to obtain unambiguous structural information that can be combined with theoretical calculations based on the known geometry and used to interpret spectroscopic and other information. A series of Ti/O clusters up to ∼2.0 nm in diameter have been synthesized and functionalized with the adsorbents catechol and isonicotinic acid. Whereas the isonicotinate is always adsorbed in a bridging monodentate mode, four different adsorption modes of catechol have been identified. The particles show a significantly larger variation of the Ti−O distances than observed in the known TiO2 phases and exhibit both sevenfold overcoordination and five- and fourfold undercoordination of the Ti atoms. Theoretical calculations show only a moderate dependence of the catecholate net charge on the geometry of adsorption. All of the catechol-functionalized clusters have a deep-red color due to penetration of the highest occupied catechol levels into the band gap of the Ti/O particles. Spectroscopic measurements of the band gap of the Ti17 cluster are in good agreement with the theoretical values and show a blue shift of ∼0.22 eV relative to those reported for anatase nanoparticles.

本研究针对结晶纳米团簇相展开系统性描述：该相指纳米级金属氧化物团簇以三维周期性阵列形式自组装而成的有序结构。经具有技术应用价值的配体功能化修饰的纳米颗粒结晶组装体，可获取明确的结构信息，这类信息可结合基于已知几何构型的理论计算，用于阐释光谱学及其他相关实验数据。研究团队已合成一系列直径约2.0 nm的钛氧（Ti/O）团簇，并分别以儿茶酚（catechol）和异烟酸（isonicotinic acid）作为吸附配体对其进行功能化修饰。实验结果显示，异烟酸根始终以桥联单齿配位模式吸附于团簇表面，研究人员已鉴定出儿茶酚的四种不同吸附构型。该类团簇中Ti-O键长的变化幅度显著大于已知二氧化钛（TiO₂）晶相中的观测值，且钛原子存在七重过配位、五重及四重欠配位的多种配位状态。理论计算结果表明，儿茶酚盐（catecholate）的净电荷仅随吸附几何构型发生中等程度的变化。所有经儿茶酚功能化修饰的团簇均呈深红色，该显色现象源于儿茶酚的最高占据分子轨道能级渗透进入钛氧团簇的带隙之中。对Ti₁₇团簇带隙的光谱测量结果与理论计算值吻合良好，且相较于锐钛矿（anatase）纳米颗粒的相关报道值，该带隙出现了约0.22 eV的蓝移。