Band structure and charge carrier dynamics in (W,N)-codoped TiO2 resolved by electrochemical impedance spectroscopy combined with UV-vis and EPR spectroscopies

A. Folli*, J. Z. Bloh, D E Macphee

*Corresponding author for this work

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

Semiconductor photocatalysis is on the verge of (probably) its most important deployment and boost since the pioneering paper of Fujishima and Honda in 1972. Photo-generation of unbound excitons, i.e. separated conduction band electrons and valence band positive holes, is the fundamental primary process triggering charge separation in solid semiconductors necessary to initiate their photocatalytic activity. Immediately after being generated, charge carriers can undergo processes like recombination, trapping in mid-band-gap states or, paramount for photocatalytic processes, transfer to species adsorbed on the solid semiconductor surface. In TiO2 and doped TiO2, interfacial charge transfers are the slowest amongst the primary processes; therefore, electron (and hole) transfer most likely occurs from single electron traps (i.e. involving radical species). We report here on an effective approach combining electrochemical impedance spectroscopy with other spectroscopic techniques such as UV-vis and electron paramagnetic resonance. This approach allows deriving important information about band structure and following electron dynamics triggered by photon absorption. The redox potentials of the band edges and the influence of the dopants on the band structure are elucidated by electrochemical impedance spectroscopy combined with UV-vis spectroscopy. Electron dynamics are then studied using electron paramagnetic resonance spectroscopy, to elucidate the photochemical reactions at the basis of the photo-generated electron-hole pairs, and subsequent trapping and/or recombination. Results of a TiO2 sample containing W and N as dopants (0.1at.% of W) highlight a narrowing of the intrinsic band gap of about 0.12eV. The semiconductor visible light photochemistry is driven by diamagnetic donor states [NiO]-, and [NiO]w- (formally NO3-), from which electrons can be excited to the conduction band, generating EPR active paramagnetic [NiO] and [NiO] w states (formally NO2-). The formation of W5+ electron trapping states, energetically more favourable than Ti3+ electron trapping states, is also identified.

Original languageEnglish
Pages (from-to)367-372
Number of pages6
JournalJournal of Electroanalytical Chemistry
Volume780
Early online date10 Nov 2015
DOIs
Publication statusPublished - 1 Nov 2016

Fingerprint

Charge carriers
Electrochemical impedance spectroscopy
Band structure
Paramagnetic resonance
Spectroscopy
Electrons
Semiconductor materials
Photochemical reactions
Conduction bands
Energy gap
Doping (additives)
Electron traps
Photocatalysis
Valence bands
Ultraviolet spectroscopy
Excitons
Charge transfer
Photons

Keywords

  • Mott-Schottky
  • Paramagnetic
  • Photocatalysis
  • Redox potential
  • Semiconductor
  • Visible light

ASJC Scopus subject areas

  • Chemical Engineering(all)
  • Analytical Chemistry
  • Electrochemistry

Cite this

@article{242d3d9f4a7d42ac9c8bdf00ced3ebb5,
title = "Band structure and charge carrier dynamics in (W,N)-codoped TiO2 resolved by electrochemical impedance spectroscopy combined with UV-vis and EPR spectroscopies",
abstract = "Semiconductor photocatalysis is on the verge of (probably) its most important deployment and boost since the pioneering paper of Fujishima and Honda in 1972. Photo-generation of unbound excitons, i.e. separated conduction band electrons and valence band positive holes, is the fundamental primary process triggering charge separation in solid semiconductors necessary to initiate their photocatalytic activity. Immediately after being generated, charge carriers can undergo processes like recombination, trapping in mid-band-gap states or, paramount for photocatalytic processes, transfer to species adsorbed on the solid semiconductor surface. In TiO2 and doped TiO2, interfacial charge transfers are the slowest amongst the primary processes; therefore, electron (and hole) transfer most likely occurs from single electron traps (i.e. involving radical species). We report here on an effective approach combining electrochemical impedance spectroscopy with other spectroscopic techniques such as UV-vis and electron paramagnetic resonance. This approach allows deriving important information about band structure and following electron dynamics triggered by photon absorption. The redox potentials of the band edges and the influence of the dopants on the band structure are elucidated by electrochemical impedance spectroscopy combined with UV-vis spectroscopy. Electron dynamics are then studied using electron paramagnetic resonance spectroscopy, to elucidate the photochemical reactions at the basis of the photo-generated electron-hole pairs, and subsequent trapping and/or recombination. Results of a TiO2 sample containing W and N as dopants (0.1at.{\%} of W) highlight a narrowing of the intrinsic band gap of about 0.12eV. The semiconductor visible light photochemistry is driven by diamagnetic donor states [NiO]-, and [NiO]w- (formally NO3-), from which electrons can be excited to the conduction band, generating EPR active paramagnetic [NiO] and [NiO] w states (formally NO2-). The formation of W5+ electron trapping states, energetically more favourable than Ti3+ electron trapping states, is also identified.",
keywords = "Mott-Schottky, Paramagnetic, Photocatalysis, Redox potential, Semiconductor, Visible light",
author = "A. Folli and Bloh, {J. Z.} and Macphee, {D E}",
note = "Acknowledgement The Authors are grateful to the European Commission for the financial support through the European Project Light2CAT. Light2CAT is funded by the European Union Seventh Framework Programme (FP7) under the grant agreement no. 283062 Eco-Innovation, Theme Environment. The authors also gratefully acknowledge the British Council and FAPESP (S{\~a}o Paulo Research Foundation) for the financial support of the workshop ELSOL — Electrochemical solutions for contemporary problems.",
year = "2016",
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doi = "10.1016/j.jelechem.2015.10.033",
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journal = "Journal of Electroanalytical Chemistry",
issn = "1572-6657",
publisher = "Elsevier BV",

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T1 - Band structure and charge carrier dynamics in (W,N)-codoped TiO2 resolved by electrochemical impedance spectroscopy combined with UV-vis and EPR spectroscopies

AU - Folli, A.

AU - Bloh, J. Z.

AU - Macphee, D E

N1 - Acknowledgement The Authors are grateful to the European Commission for the financial support through the European Project Light2CAT. Light2CAT is funded by the European Union Seventh Framework Programme (FP7) under the grant agreement no. 283062 Eco-Innovation, Theme Environment. The authors also gratefully acknowledge the British Council and FAPESP (São Paulo Research Foundation) for the financial support of the workshop ELSOL — Electrochemical solutions for contemporary problems.

PY - 2016/11/1

Y1 - 2016/11/1

N2 - Semiconductor photocatalysis is on the verge of (probably) its most important deployment and boost since the pioneering paper of Fujishima and Honda in 1972. Photo-generation of unbound excitons, i.e. separated conduction band electrons and valence band positive holes, is the fundamental primary process triggering charge separation in solid semiconductors necessary to initiate their photocatalytic activity. Immediately after being generated, charge carriers can undergo processes like recombination, trapping in mid-band-gap states or, paramount for photocatalytic processes, transfer to species adsorbed on the solid semiconductor surface. In TiO2 and doped TiO2, interfacial charge transfers are the slowest amongst the primary processes; therefore, electron (and hole) transfer most likely occurs from single electron traps (i.e. involving radical species). We report here on an effective approach combining electrochemical impedance spectroscopy with other spectroscopic techniques such as UV-vis and electron paramagnetic resonance. This approach allows deriving important information about band structure and following electron dynamics triggered by photon absorption. The redox potentials of the band edges and the influence of the dopants on the band structure are elucidated by electrochemical impedance spectroscopy combined with UV-vis spectroscopy. Electron dynamics are then studied using electron paramagnetic resonance spectroscopy, to elucidate the photochemical reactions at the basis of the photo-generated electron-hole pairs, and subsequent trapping and/or recombination. Results of a TiO2 sample containing W and N as dopants (0.1at.% of W) highlight a narrowing of the intrinsic band gap of about 0.12eV. The semiconductor visible light photochemistry is driven by diamagnetic donor states [NiO]-, and [NiO]w- (formally NO3-), from which electrons can be excited to the conduction band, generating EPR active paramagnetic [NiO] and [NiO] w states (formally NO2-). The formation of W5+ electron trapping states, energetically more favourable than Ti3+ electron trapping states, is also identified.

AB - Semiconductor photocatalysis is on the verge of (probably) its most important deployment and boost since the pioneering paper of Fujishima and Honda in 1972. Photo-generation of unbound excitons, i.e. separated conduction band electrons and valence band positive holes, is the fundamental primary process triggering charge separation in solid semiconductors necessary to initiate their photocatalytic activity. Immediately after being generated, charge carriers can undergo processes like recombination, trapping in mid-band-gap states or, paramount for photocatalytic processes, transfer to species adsorbed on the solid semiconductor surface. In TiO2 and doped TiO2, interfacial charge transfers are the slowest amongst the primary processes; therefore, electron (and hole) transfer most likely occurs from single electron traps (i.e. involving radical species). We report here on an effective approach combining electrochemical impedance spectroscopy with other spectroscopic techniques such as UV-vis and electron paramagnetic resonance. This approach allows deriving important information about band structure and following electron dynamics triggered by photon absorption. The redox potentials of the band edges and the influence of the dopants on the band structure are elucidated by electrochemical impedance spectroscopy combined with UV-vis spectroscopy. Electron dynamics are then studied using electron paramagnetic resonance spectroscopy, to elucidate the photochemical reactions at the basis of the photo-generated electron-hole pairs, and subsequent trapping and/or recombination. Results of a TiO2 sample containing W and N as dopants (0.1at.% of W) highlight a narrowing of the intrinsic band gap of about 0.12eV. The semiconductor visible light photochemistry is driven by diamagnetic donor states [NiO]-, and [NiO]w- (formally NO3-), from which electrons can be excited to the conduction band, generating EPR active paramagnetic [NiO] and [NiO] w states (formally NO2-). The formation of W5+ electron trapping states, energetically more favourable than Ti3+ electron trapping states, is also identified.

KW - Mott-Schottky

KW - Paramagnetic

KW - Photocatalysis

KW - Redox potential

KW - Semiconductor

KW - Visible light

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JO - Journal of Electroanalytical Chemistry

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