Experimental and Theoretical Investigations on the Activity and Stability of Substitutional and Interstitial Boron in TiO₂ Photocatalyst

Abstract

Effects of boron doped in TiO2 at (a) interstitial site (Bint), (b) substitutional site (Bsub), and (c) combination of both the sites (Bint+sub) have been investigated experimentally and theoretically to understand the origin of enhanced photocatalytic activity and stability. B-doped TiO2 powders were synthesized by sol–gel method with different concentrations of boron. XPS results indicate that boron first prefers Bint site when doped with low concentration (up to 1 at. % B), but as the concentration increases (2 at. % and above) B also occupies substitutional O position in addition to Bint to form TiO2 containing Bint+sub (TiO2–Bint+sub). Higher absorption of visible light is achieved for TiO2–Bint+sub due to the presence of two absorption edges (2.4 and 2.2 eV) as observed in the absorption spectra, while insignificant narrowing of band gap is observed for TiO2–Bint. Electronic structure calculated by DFT for TiO2 with Bint, Bsub, and Bint+sub revealed that the two localized deep levels are formed in the mid gap region which are responsible for these optical transitions for TiO2–Bint+sub. Photoluminescence (PL) emission spectra showed that the shallow level (as inferred from the DFT calculations) created below the conduction band is able to decrease the radiative recombination process in TiO2–Bint by trapping electrons and prolonging the lifetime of charge carriers as observed in the time-resolved PL decay curve. Furthermore, lower effective mass ratio of charge carriers calculated using DFT for TiO2–Bint also suggests better charge mobility and low recombination rate. Photocatalytic degradation rate of organic pollutants in water was significantly higher after B-doping with higher performance obtained with TiO2 containing Bint as compared to Bint+sub. By imposing the destabilizing circumstances it was established that TiO2–Bsub is metastable and collapses under mild conditions, whereas TiO2–Bint is highly stable and retains all its properties. All these unprecedented findings disclose that higher activity of TiO2–Bint as compared to that of TiO2–Bint+sub is mainly because of the delayed recombination processes even though the optical band gap is not significantly varied.