title: Where Do Photogenerated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime.
abstract: Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood. Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase composition. By exploiting the different signatures in the photoinduced absorption of trapped charges in anatase and rutile, we were able to separately track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photogenerated holes transfer from rutile to anatase on submicrosecond time scales. This hole transfer can significantly increase the anatase hole yield, with a 20:80 anatase:rutile composite showing a 5-fold increase in anatase holes observed from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irrespective of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for determining the direction of photogenerated charge separation across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.
authors.affiliation: Department of Chemistry, Imperial College London , South Kensington Campus, London SW7 2AZ, U.K.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kafizas
authors.affiliation: Department of Chemistry, Imperial College London , South Kensington Campus, London SW7 2AZ, U.K.|State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences , 457 Zhongshan Road, Dalian 116023, China.
authors.forename: Xiuli
authors.initials: X
authors.lastname: Wang
authors.affiliation: Department of Chemistry, Imperial College London , South Kensington Campus, London SW7 2AZ, U.K.
authors.forename: Stephanie R
authors.initials: SR
authors.lastname: Pendlebury
authors.affiliation: Department of Chemistry, Imperial College London , South Kensington Campus, London SW7 2AZ, U.K.
authors.forename: Piers
authors.initials: P
authors.lastname: Barnes
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Min
authors.initials: M
authors.lastname: Ling
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Carlos
authors.initials: C
authors.lastname: Sotelo-Vazquez
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Raul
authors.initials: R
authors.lastname: Quesada-Cabrera
authors.affiliation: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences , 457 Zhongshan Road, Dalian 116023, China.
authors.forename: Can
authors.initials: C
authors.lastname: Li
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Ivan P
authors.initials: IP
authors.lastname: Parkin
authors.affiliation: Department of Chemistry, Imperial College London , South Kensington Campus, London SW7 2AZ, U.K.
authors.forename: James R
authors.initials: JR
authors.lastname: Durrant