The influence of halides in polyoxotitanate cages; dipole moment, splitting and expansion of d-orbitals and electron–electron repulsion

Schirin Hanf,Peter D. Matthews,Dominic S. Wright,Ning Li,He-Kuan Luo

doi:10.1039/c6dt04288k

Abstract

Metal-doped polyoxotitanate (M-POT) cages have been shown to be efficient single-source precursors to metal-doped titania [TiO2(M)] (state-of-the-art photocatalytic materials) as well as molecular models for the behaviour of dopant metal ions in bulk titania. Here we report the influence halide ions have on the optical and electronic properties of a series of halide-only, and cobalt halide-'doped' POT cages. In this combined experimental and computational study we show that halide ions can have several effects on the band gaps of halide-containing POT cages, influencing the dipole moment (hole-electron separation) and the structure of the valance band edge. Overall, the band gap behaviour stems from the effects of increasing orbital energy moving from F to I down Group 17, as well as crystal-field splitting of the d-orbitals, the potential effects of the Nephelauxetic influence of the halides and electron-electron repulsion.

Highlights

Metal-‘doped’ polyoxotitanate (M-POT) cages of the form [TixOy(OR)zMnXm] (R = Et or iPr, M = transition metal or lanthanide and X = halide) have been of great interest in recent years as single-source precursors to metal-doped TiO2 [TiO2(M)].1–11 In undoped TiO2, excitation leads to the promotion of an electron from the valence band [ predominately O(-p)] to the conduction band [predominately Ti(-d)].12–15 The introduction of a metal dopant into TiO2 can improve the quantum efficiency of this process and the ability to harness a broader range of solar energy in two ways
We report the influence halide ions have on the optical and electronic properties of a series of halide-only, and cobalt halide-‘doped’ POT cages. In this combined experimental and computational study we show that halide ions can have several effects on the band gaps of halide-containing POT cages, influencing the dipole moment and the structure of the valance band edge
The focus of this current work was on the effects of halide ions on the band gaps of ‘undoped’ polyoxotitanate (POT) and metal-‘doped’ polyoxotitanate cages (M-POT)

Summary

Introduction

In this combined experimental and computational study we show that halide ions can have several effects on the band gaps of halide-containing POT cages, influencing the dipole moment (hole–electron separation) and the structure of the valance band edge. Paper the ‘undoped’ cage 1 exhibits a larger band gap than the Cl-‘doped’ counterpart 1-Cl. DFT calculations were carried out to predict the influence of the halide on the band gaps of the whole series 1-X (X = F, Cl, Br, I).

Results

Conclusion