Investigating interfacial electron transfer in
dye-sensitized NiO using vibrational spectroscopy

Black, Fiona A.; Clark, Charlotte A.; Summers, Gareth H.; Clark, Ian P.; Towrie, Michael; Penfold, Thomas J.; George, Michael W.; Gibson, Elizabeth A.

Abstract

Understanding what influences the formation and lifetime of charge-separated states is key to developing photoelectrochemical devices. This paper describes the use of time-resolved infrared absorption spectroscopy (TRIR) to determine the structure and lifetime of the intermediates formed on photoexcitation of two organic donor–π–acceptor dyes adsorbed to the surface of NiO. The donor and π-linker of both dyes is triphenylamine and thiophene but the acceptors differ, maleonitrile (1) and bodipy (2). Despite their structural similarities, dye 1 outperforms 2 significantly in devices. Strong transient bands in the fingerprint region (1 and 2) and nitrile region (2300–2000 cm−1) for 1 enabled us to monitor the structure of the excited states in solution or adsorbed on NiO (in the absence and presence of electrolyte) and the corresponding kinetics, which are on a ps–ns timescale. The results are consistent with rapid (<1 ps) charge-transfer from NiO to the excited dye (1) to give exclusively the charge-separated state on the timescale of our measurements. Conversely, the TRIR experiments revealed that multiple species are present shortly after excitation of the bodipy chromophore in 2, which is electronically decoupled from the thiophene linker. In solution, excitation first populates the bodipy singlet excited state, followed by charge transfer from the triphenylamine to the bodipy. The presence and short lifetime (τ ≈ 30 ps) of the charge-transfer excited state when 2 is adsorbed on NiO (2|NiO) suggests that charge separation is slower and/or less efficient in 2|NiO than in 1|NiO. This is consistent with the difference in performance between the two dyes in dye-sensitized solar cells and photoelectrochemical water splitting devices. Compared to n-type materials such as TiO2, less is understood regarding electron transfer between dyes and p-type metal oxides such as NiO, but it is evident that fast charge-recombination presents a limit to the performance of photocathodes. This is also a major challenge to photocatalytic systems based on a “Z-scheme”, where the catalysis takes place on a µs–s timescale.

Citation

dye-sensitized NiO using vibrational spectroscopy. Physical Chemistry Chemical Physics, 19(11), 7877-7885. https://doi.org/10.1039/c6cp05712h

• Journal Article Type: Article
• Acceptance Date: Feb 22, 2017
• Publication Date: Feb 22, 2017
• Deposit Date: Sep 11, 2017
• Publicly Available Date: Sep 11, 2017
• Journal: Physical Chemistry Chemical Physics
• Print ISSN: 1463-9076
• Electronic ISSN: 1463-9084
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 19
• Issue: 11
• Pages: 7877-7885
• DOI: https://doi.org/10.1039/c6cp05712h
• Public URL: https://nottingham-repository.worktribe.com/output/844689
• Publisher URL: http://pubs.rsc.org/en/content/articlelanding/2017/cp/c6cp05712h#!divAbstract
• Additional Information: : This document is Similarity Check deposited; : Supplementary Information; : Elizabeth A. Gibson (ORCID); : The PCCP Owner Societies have an exclusive publication licence for this journal; OPEN ACCESS: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0); : Single-blind; : Received 17 August 2016; Accepted 22 February 2017; Accepted Manuscript published 22 February 2017; Advance Article published 6 March 2017; Version of Record published 15 March 2017
• Contract Date: Sep 11, 2017