Revealing the true impact of interstitial and substitutional nitrogen doping in TiO2 on photoelectrochemical applications

Khan, Sherdil; Lemes Ruwer, Thais; Khan, Niqab; K�che, Ariadne; Lodge, Rhys W.; Coelho-J�nior, Hor�cio; Sommer, Rubem L.; Leite Santos, Marcos J.; Malfatti, C�lia F.; Bergmann, Carlos P.; Fernandes, Jesum Alves

Abstract

Application of photocatalysts that strongly absorb within the visible range is a common strategy to improve the efficiency of photoelectrochemical (PEC) systems; this may translate to high photocurrents, but it is not always the case. Here, we show that nitrogen doping enhances visible light absorption of TiO2; however, it does not necessarily result in improved PEC performance. Depending on the applied external potential, N-doping can improve, or degrade, PEC performance either under water oxidation conditions or via hole scavenging (Na2S/Na2SO3). In this work, we developed a holistic approach to evaluate the true impact of N doping in TiO2 on PEC performance. Interstitial and substitutional N doping are experimentally explored for the first time through a simple and novel PEC approach which complemented X-ray photoelectron analyses. Using this approach, we show that interstitial N doping of anatase TiO2 dominates up to 400 °C and substitutional doping up to ca. 600 °C, without rutile formation. This reveals that the bottleneck for doping higher N-concentrations in TiO2 is the direct transformation to thermodynamically favorable N-rich phases, such as TiN/Ti2N at 700 °C, inhibiting the formation of rutile phase. Transmission electron microscopy revealed that N doping proceeds mainly from the inner to the outer tube walls via nitridation and follows a preferential pathway from interstitial to substitutional doping. Direct PEC experimental evidence on visible light activation of N doped TiO2, and the location of interband states, showed acceptor levels of 1.0 eV for substitutional and 0.7 eV for interstitial doping above the TiO2 valence band maximum. In addition, due to O vacancies and Ti3+ species, donor levels below the conduction band minimum were also created. These levels act as trapping/recombination centers for charge carriers and, therefore, the gain in the visible range due to N doping does not translate to an improved PEC performance by these structural defects. Ultimately, we show that whilst there is a benefit of visible light absorption through N doping in TiO2, the PEC performance of the samples only surpasses pristine TiO2 at relatively high biasing (>0.3 V vs. Ag/AgCl).

Citation

Khan, S., Lemes Ruwer, T., Khan, N., Köche, A., Lodge, R. W., Coelho-Júnior, H., …Fernandes, J. A. (2021). Revealing the true impact of interstitial and substitutional nitrogen doping in TiO2 on photoelectrochemical applications. Journal of Materials Chemistry A, 9(20), 12214-12224. https://doi.org/10.1039/d0ta11494d

• Journal Article Type: Article
• Acceptance Date: Apr 29, 2021
• Online Publication Date: Apr 30, 2021
• Publication Date: May 28, 2021
• Deposit Date: Jun 24, 2021
• Publicly Available Date: Jun 25, 2021
• Journal: Journal of Materials Chemistry A
• Print ISSN: 2050-7488
• Electronic ISSN: 2050-7496
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 9
• Issue: 20
• Pages: 12214-12224
• DOI: https://doi.org/10.1039/d0ta11494d
• Keywords: Renewable Energy, Sustainability and the Environment; General Materials Science; General Chemistry
• Public URL: https://nottingham-repository.worktribe.com/output/5535651
• Publisher URL: https://pubs.rsc.org/en/content/articlelanding/2021/TA/D0TA11494D#!divAbstract
• Additional Information: : This document is Similarity Check deposited; : Supplementary Information; : Sherdil Khan (ORCID); : Sherdil Khan (ResearcherID); : Ariadne Köche (ORCID); : Horácio Coelho-Júnior (ORCID); : Marcos J. Leite Santos (ORCID); : Jesum Alves Fernandes (ORCID); : Jesum Alves Fernandes (ResearcherID); : Single-blind; : Received 25 November 2020; Accepted 29 April 2021; Accepted Manuscript published 30 April 2021; Advance Article published 13 May 2021