Solar Energy & Advanced Materials Research Group


New Paper in Journal of the American Chemical Society

28 March 2017

 We are pleased to announce the new paper 'Time-Resolved Spectroscopic Investigation of Charge Trapping in Carbon Nitrides Photocatalysts for Hydrogen Generation' published by our group members (Yiou Wang, Junwang Tang) and our collabrators in Imperial College (James Durrant et.

jacs al) in Journal of the American Chemical Society.

It is now avilable online: http://pubs.acs.org/doi/abs/10.1021/jacs.7b01547.


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 timescales ranging from femtoseconds to seconds. Free charge carriers form within a 200 fs excitation pulse, trap on the picosecond timescale 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 timescales 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 optimise photocatalytic activity of these materials is to prevent electron trapping into deep, and photocatalytically inactive, electron trap states.