posted on 2022-12-22, 14:35authored byChang Liu, Jian Liu, Robert Godin
The photocatalytic hydrogen evolution reaction is one
of the most
promising approaches to utilize solar energy and produce hydrogen
as an alternative energy source to traditional fossil fuels. In this
work, we focused on further exploring the cocatalyst potential of
the transition metal nickel and prepared nickel oxide/carbon nitride
(NiO/CNx) heterojunctions through thermal
polymerization as well as plasma-enhanced atomic layer deposition
(PEALD). The optimal NiO loading thickness on the surface of the CNx was found to be 6.5 nm, prepared from 65
cycles of ALD. This NiO(65)/CNx had a
photocatalytic hydrogen evolution rate of 3.9 μmol h–1 in a 2 mg mL–1 aqueous photocatalyst solution
(195 μmol h–1 g–1), corresponding
to an AQY of 3.1% and exhibiting 54% of the activity of 3 wt % Pt/CNx under 405 nm light illumination with a sacrificial
reagent. A 3 wt % Ni/CNx prepared through
a photodeposition process from a dispersion of CNx and simple NiCl2 had a photocatalytic hydrogen
evolution rate of 2.8 μmol h–1 (140 μmol
h g–1) and an AQY of 2.2% with 2 mg mL–1 photocatalyst in the reactant solution, corresponding to 39% of
the activity of 3 wt % Pt/CNx. An induction
period at the beginning of the hydrogen evolution process was found
in all Ni-based photocatalysts, which we related to a Ni in
situ photoreduction from NiII to Ni0 and the reverse reaction. In addition, the reproducibility of the
induction period even in an oxygen-free environment suggests that
electrons flow back from reduced Ni species to CNx under dark conditions. Furthermore, a large population of
trap states was detected in the bulk CNx materials via photoinduced absorption spectroscopy, and the trapping
severely hinders cocatalysts (even the benchmark Pt cocatalyst) from
extracting charges from CNx for subsequent
interfacial redox reactions. We conclude that not only does the surface/interface
engineering of the photocatalyst/catalyst heterojunctions need to
be considered but also trap states in CNx that severely limit the transfer of photogenerated charges to cocatalysts.