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

Abstract

Effects of boron doped in TiO 2 at a) interstitial site (B int ); b) substitutional site (B sub ) and combination of both the sites (B int+sub ), have been investigated experimentally and theoretically to understand the origin of enhanced photocatalytic activity and stability. Bdoped TiO 2 powder was synthesized by sol-gel method with different concentrations of boron. XPS results indicate that boron first prefers B int site when doped with low concentration (upto 1 at.% B), but as the concentration increases (2 at.% and above) B also occupies substitutional O position in addition to B int to form TiO 2 containing B int+sub (TiO 2 -B int+sub ). Higher absorption of visible light is achieved for TiO 2 -B int+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 TiO 2 -B int . Electronic structure calculated by DFT for TiO 2 with B int , B sub , and B int+sub revealed that the two localized deep levels are formed in the mid gap which are responsible for these optical transitions for TiO 2 -B int+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 TiO 2 -B int 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 TiO 2 -B int also suggests better charge mobility and low recombination rate. Photocatalytic degradation rate of organic pollutant in water was significantly higher after B-doping with higher performance obtained with TiO 2 containing B int as compared to B int+sub . By imposing the destabilizing circumstances it was established that TiO 2 -B sub is metastable and collapses under mild conditions, whereas TiO 2 -B int is highly stable and retains all its properties. All these unprecedented findings disclose that higher activity of TiO 2 -B int as compared to that of TiO 2 -B int+sub is mainly because of the delayed recombination processes even though the optical band gap is not significantly varied.

Interstitial Boron-Doped TiO2 Thin Films: The Significant Effect of Boron on TiO2 Coatings Grown by Atmospheric Pressure Chemical Vapor Deposition

“…Several cycles of cleaning and UV irradiation did not involve the loss of boron. This stability is due to the interstitial occupancy of boron in the TiO2 lattice, fact which has been reported and studied previously by N. Patel et al25…”

“…Furthermore, previous calculations by DFT, suggested better mobility and low recombination rate for interstitial B-TiO2. 25 Another significant distinction from the work by Carmichael et al is the crystallinity enhancement of the TiO2 thin film when doped with interstitial boron. While, O-substitutional boron-doping of APCVD TiO2 was reported to imply a crystallinity devaluation 26 (i.e.…”

Interstitial boron-doped anatase TiO2 thin-films on optical fibres: atmospheric pressure-plasma enhanced chemical vapour deposition as the key for functional oxide coatings on temperature-sensitive substrates

“…Also, developing the doping process by AP-PECVD, and using different types of dopants could enable doped-TiO 2 to perform under visible light, as reported previously. 13,26 Cross-sectional SEM analysis showed the effective deposition of B-TiO 2 thin lms on OF substrates ( Fig. 1d and e).…”

“…12 Interestingly, when boron occupies an interstitial position within the TiO 2 lattice, the stability of the boron-doped TiO 2 is far superior to that of substitutional boron, because, in terms of formation energy per unit cell, B int is the highly preferred B-doping site in TiO 2 , while B sub is difficult to obtain experimentally because of the high formation energy. 13 The substitutional boron-doped TiO 2 , which seems to be metastable, decomposes into boron oxide. [11][12][13][14] TiO 2 thin lm coatings have attracted the attention of scientists for the past few years and have been commonly produced using different synthetic methods, such as hydrothermal treatment, 15 sol-gel, 16 surface impregnation, electrochemical deposition, 17 and physical 18 and chemical vapour deposition methods.…”

“…13 The substitutional boron-doped TiO 2 , which seems to be metastable, decomposes into boron oxide. [11][12][13][14] TiO 2 thin lm coatings have attracted the attention of scientists for the past few years and have been commonly produced using different synthetic methods, such as hydrothermal treatment, 15 sol-gel, 16 surface impregnation, electrochemical deposition, 17 and physical 18 and chemical vapour deposition methods. 19 Chemical vapour deposition (CVD) of thin lms and surface coatings is a well-known and versatile technique employed to synthesize a vast number of metal and non-metal chemical species.…”