Time-resolved spectroscopic investigation of charge trapping in carbon nitrides photocatalysts for hydrogen generation

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Title: Time-resolved spectroscopic investigation of charge trapping in carbon nitrides photocatalysts for hydrogen generation
Author(s): Godin, RP
Wang, Y
Zwijnenburg, MA
Tang, J
Durrant, J
Item Type: Journal Article
Abstract: Carbon nitride (g-C3N4) as a benchmark polymer photocatalyst is attracting significant research interest because of its visible light photocatalytic performance combined with good stability and facile synthesis. However, little is known about the fundamental photophysical processes of g-C3N4, which are key to explain and promote photoactivity. Using time-resolved absorption and photoluminescence spectroscopies, we have investigated the photophysics of a series of carbon nitrides on time scales ranging from femtoseconds to seconds. Free charge carriers form within a 200 fs excitation pulse, trap on the picosecond time scale with trap states in a range of energies, and then recombine with power law decays that are indicative of charge trapping–detrapping processes. Delayed photoluminescence is assigned to thermal excitation of trapped carriers back up to the conduction/valence bands. We develop a simple, quantitative model for the charge carrier dynamics in these photocatalysts, which includes carrier relaxation into an exponential tail of trap states extending up to 1.5 eV into the bandgap. This trapping reduces the efficiency of surface photocatalytic reactions. Deep trapped electrons observed on micro- to millisecond time scales are unable to reduce electron acceptors on the surface or in solution. Within a series of g-C3N4, the yield of these unreactive trapped electrons correlates inversely with H2 evolution rates. We conclude by arguing that the photophysics of these carbon nitride materials show closer parallels with inorganic semiconductors than conjugated polymers, and that the key challenge to optimize photocatalytic activity of these materials is to prevent electron trapping into deep, and photocatalytically inactive, electron trap states.
Publication Date: 20-Mar-2017
Date of Acceptance: 20-Mar-2017
URI: http://hdl.handle.net/10044/1/45763
DOI: https://dx.doi.org/10.1021/jacs.7b01547
ISSN: 1520-5126
Publisher: American Chemical Society
Start Page: 5216
End Page: 5224
Journal / Book Title: Journal of the American Chemical Society
Volume: 139
Issue: 14
Copyright Statement: © 2017 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited (http://pubs.acs.org/page/policy/authorchoice_ccby_termsofuse.html)
Sponsor/Funder: Commission of the European Communities
Funder's Grant Number: 291482
Keywords: Science & Technology
Physical Sciences
Chemistry, Multidisciplinary
General Chemistry
03 Chemical Sciences
Publication Status: Published
Appears in Collections:Chemistry
Faculty of Natural Sciences

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