Precise and Wide-Ranged Band-Gap Tuning of Ti₆-Core-Based Titanium Oxo Clusters by the Type and Number of Chromophore Ligands

Abstract

Sensitization with dyes is the most promising option to narrow the band gap and improve visible-light absorption of TiO₂. As ideal structure and reaction models of TiO₂, titanium oxo clusters (TOCs) with exact crystal structure are beneficial for further understanding the structure-property relationship of TiO₂. Most reports mainly focus on the synthesis and properties of TOCs, but research on the band-gap tuning of TOCs at the large range of 3.7-2.0 eV by chromophore ligands (CLs) has been lacking. Herein, six new Ti₆-core-based TOCs (Ti₆-TOCs) were successfully synthesized by using CLs in a simple and general approach. Each Ti₆-TOC structure contains two Ti₃(μ₃-O) units featuring a flat or pyramidal mode as building blocks. Five Ti₆-core structure types were present among the six Ti₆-TOCs, which enriched the family of hexanuclear TOCs. To be noted, the band-gap values were tuned at a wide range of 3.41-1.98 eV by controlling the type and number of the CLs 2-hydroxypyridine, salicylaldoxime, and catechol in the structure. Density functional theory calculation revealed that the lowest-energy bands of these Ti₆-TOCs are attributed to the CL-to-TiO core charge-transfer bands. This work provides a precise and wide-ranged band-gap tuning method for TOCs. Additionally, the six Ti₆-TOCs exhibit good photocurrent response.