ALD-Deposited NiO Approaches the Performance of Platinum as a Hydrogen Evolution Cocatalyst on Carbon Nitride

Abstract

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/CN x ) heterojunctions through thermal polymerization as well as plasma-enhanced atomic layer deposition (PEALD). The optimal NiO loading thickness on the surface of the CN x was found to be 6.5 nm, prepared from 65 cycles of ALD. This NiO(65)/CN x 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/CN x under 405 nm light illumination with a sacrificial reagent. A 3 wt % Ni/CN x prepared through a photodeposition process from a dispersion of CN x and simple NiCl 2 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/CN x . 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 Ni II to Ni 0 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 CN x under dark conditions. Furthermore, a large population of trap states was detected in the bulk CN x materials via photoinduced absorption spectroscopy, and the trapping severely hinders cocatalysts (even the benchmark Pt cocatalyst) from extracting charges from CN x 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 CN x that severely limit the transfer of photogenerated charges to cocatalysts.

Aminosilanized Interface Promotes Electrochemically Stable Carbon Nitride Films with Fewer Trap States on FTO for (Photo)electrochemical Systems

“…We first attempted to prepare CN x /FTO electrodes following the procedure in our previous work . The FTO glass was dipped in saturated thiourea solution to crystallize the CN x precursor on the surface.…”

“…We first attempted to prepare CN x /FTO electrodes following the procedure in our previous work. 25 The FTO glass was dipped in saturated thiourea solution to crystallize the CN x precursor on the surface. Thiourea was chosen as the CN x precursor since it is reported that sulfur atoms could work as the molten mediator to initialize the growth of the CN x on the FTO glass.…”

Emerging Atomic Layer Deposition Technology toward High‐k Gate Dielectrics, Energy, and Photocatalysis Applications

“…ALD, as a state-of-the-art technology, can achieve uniform deposition on complex 3D substrates with conformal morphology, which has broad application prospects in the energy field. For example, ALD TiO 2 or NiO on g-C 3 N 4 for photocatalytic hydrogen production, [40,41] as well as ALD TiO 2 thin films, can protect silicon-based hybrid photoelectrodes for photoelectrochemical hydrogen production. [42] In terms of hydrogen sensing, ALD can coat oxide sensing layers and prepare hydrogen sensing optical fibers.…”