Optical properties of anatase TiO2: synergy between transition metal doping and oxygen vacancies

Abstract

Charge carriers (electrons and holes) are generated on the TiO using UV radiation; this excitation energy can be reduced by modifying the material electronic structure, for example, by doping or creating oxygen vacancies. Here, the electronic structure of a transition metal-doped anatase, bulk and surface, and their interaction with oxygen vacancies are studied using density functional theory. The visible light response of metal-doped TiO (101) is also determined. Transition metals generate intra-band gap states, which reduce the excitation energy but may also act as charge recombination sites. Dopants Fe, Co, and Ni remarkably enhance the visible light response due to the states in the middle of the gap. However, Co and Ni create heavier charge carriers. Our results show that Pd and Pt-doped TiO generate states near the valence and conduction band with a "clean" band gap (without states in the middle of the gap). Moreover, Pt-doped TiO maintains the charge mobility because it presents a small charge carriers mass. Hence, Pt-doped TiO represents the best alternative to activate TiO under visible light. The optical response of transition metal-doped TiO follows the order 3d > 4d > 5d. The oxygen vacancies reduce the response of metal-doped TiO to visible light because the unpaired electrons generated occupy the empty states of metal-doping. Graphical Abstract Density of states of TiO (101) surface doped with transition metals and oxygen vacancies.

Greatly Improving Electrochemical N2 Reduction over TiO2 Nanoparticles by Iron Doping

“…XRD and HRTEM indicate that the (101) surface is mainly exposed in the Fe‐TiO 2 system and is thus applied to catalyze the NRR. In general, the Fe atom substitutes the five‐coordinate Ti atom on the TiO 2 (101) surface and creation of oxygen vacancies (V) becomes more probable because of the charge compensation between the metal dopant and lattice defects . Our calculations show that the V 1 site near the Fe dopant atom is most probable on an Fe‐TiO 2 (101) surface with an oxygen vacancy formation energy of 1.42 eV ( E f =1.42 eV), which is lower than that of a V 1 ‐decorated pristine TiO 2 (101) surface with E f =3.70 eV (Figure S14).…”

“…The latter presents a definite oxygen vacancy signal located at g =2.003, demonstrating that a large number of oxygen vacancies are formed in the wake of the Ti 3+ centers . The unpaired electrons from V 1 ‐decorated Fe‐TiO 2 present an open‐shell singlet configuration (↑↓), indicating the absence of Ti 3+ (Figure S16 a) . Given that Ti 3+ exists in this catalyst, the second oxygen vacancy is further considered and the corresponding three possible vacancy sites are selected; including, V 2‐1 , V 2‐2 , and V 2‐3 (Figure S14).…”

Greatly Improving Electrochemical N2 Reduction over TiO2 Nanoparticles by Iron Doping

“…[52] Our calculations show that the V 1 site near the Fe dopant atom is most probable on an Fe-TiO 2 (101) surface with an oxygen vacancy formation energy of 1.42 eV (E f = 1.42 eV), which is lower than that of aV 1 -decorated pristine TiO 2 (101) surface with E f = 3.70 eV ( Figure S14). In general, the Fe atom substitutes the five-coordinate Ti atom on the TiO 2 (101) surface and creation of oxygen vacancies (V) becomes more probable because of the charge compensation between the metal dopant and lattice defects.…”

“…[52] Our calculations show that the V 1 site near the Fe dopant atom is most probable on an Fe-TiO 2 (101) surface with an oxygen vacancy formation energy of 1.42 eV (E f = 1.42 eV), which is lower than that of aV 1 -decorated pristine TiO 2 (101) surface with E f = 3.70 eV ( Figure S14). [52] Given that Ti 3+ exists in this catalyst, the second oxygen vacancyi s further considered and the corresponding three possible vacancy sites are selected;i ncluding,V 2-1 ,V 2-2 ,a nd V 2-3 ( Figure S14). Figure S15 shows the room temperature electron spin resonance (ESR) spectra of TiO 2 and Fe-TiO 2 .T he latter presents ad efinite oxygen vacancys ignal located at g = 2.003, demonstrating that al arge number of oxygen vacancies are formed in the wake of the Ti 3+ centers.…”

Strongly Bound Frenkel Excitons on TiO2 Nanoparticles: An Evolutionary and DFT Approach

“…Conventional DFT methods typically underestimated the fundamental gap of insulators and semiconductors by 40% due to the derivative discontinuity of the exact exchange-correlation energy [41]. For instance, an underestimation of around 28% for anatase TiO 2 is reported using the generalized gradient approximation (GGA) exchange-correlation functionals [42,43].…”