Schottky-Barrier-Free Plasmonic Photocatalysts for Solar-Driven Ammonia Synthesis
Authors: Wu Boyuan , Wang Jianfang 
Published: August 2024

Ammonia is not only an indispensable feedstock to produce essential industrial chemicals like fertilizers, but it is also a potential hydrogen energy carrier owing to its high energy density and hydrogen capacity. Currently, most ammonia required for humans comes from the Haber–Bosch process, which contributes more than 90% of the annual global ammonia output. However, the harsh reaction conditions of this process result in serious energy and environmental issues. According to the relevant statistics, the ammonia industry consumes about 2% of the total energy and generates about 1.6% of the total CO2 emission annually in the world.

It is therefore crucial to develop a green and sustainable strategy for ammonia synthesis. Among various alternative technologies of the Haber–Bosch process, artificial photosynthesis has attracted wide research interests and has been regarded as a promising, environmentally friendly, and energy-saving process. Photocatalytic ammonia synthesis can utilize renewable solar energy to drive the reaction and operate under mild conditions, which possesses the benefits of the simple reaction system, low cost, and safety. Although numerous photocatalysts have been explored over the past several decades, their activity and efficiency have still been low and far from the practical application requirements. It is therefore a huge challenge to design efficient photocatalysts for ammonia synthesis.

Localized surface plasmon resonance (LSPR) has recently drawn enormous attention. LSPR can not only endow plasmonic nanomaterials with large absorption cross-sections and intense local field enhancement but also generate energetic hot electrons and hot holes. Owing to the appealing features of LSPR, plasmonic photocatalysis has shown great potential in ammonia synthesis. Plasmonic photocatalysts are usually composed of noble metals and wide bandgap semiconductors. However, the Schottky barrier will form at the metal–semiconductor interface in such hybrid structures due to the mismatch of the band structures between plasmonic metals and semiconductors. Hence, a large portion of the hot electrons cannot cross the Schottky barrier and get lost in the metal through the recombination with the holes. To overcome the drawbacks caused by the Schottky barrier, it is strongly desirable to develop a new type of plasmonic semiconductor photocatalysts, i.e., Schottky-barrier-free plasmonic photocatalyst.

The Schottky-barrier-free plasmonic photocatalysts are usually degenerately doped semiconductors containing lattice oxygen vacancies. They possess high free electron densities and strong plasmon resonance because of the high-level doping, which results in abundant hot charge carriers upon plasmon excitation. Moreover, they also have appropriate surface sites that can adsorb reactant molecules and activate them. The photogenerated plasmonic hot charge carriers can get trapped at the defect sites and thereafter drive redox reactions. Compared with traditional metal–semiconductor hybrid plasmonic photocatalysts, Schottky-barrier-free plasmonic photocatalysts enable hot charge carriers to be transported nearly freely in the material, resulting in high charge utilization and photocatalytic efficiency.

We have demonstrated the synthesis of Schottky-barrier-free plasmonic MoO3−x
nanospheres (NSs) with remarkably high oxygen vacancy (OV) contents by aerosol-spray. The MoO3−x
NSs can achieve efficient ammonia photosynthesis through a “killing two birds with one stone” mechanism. On the one hand, the OVs can serve as chemisorption sites for N2 molecules and participate in their activation. On the other hand, the introduction of OVs generates a large number of free electrons near the conduction band, which induces a broad absorption band from the visible to the NIR region owing to the plasmon resonance. The Schottky-barrier-free nature of this plasmonic semiconductor allows free transportation of the plasmonic hot charge carriers, while the defect states introduced by the OVs can effectively trap the hot electrons to prevent rapid electron–hole recombination. As expected, the as-prepared material exhibits a superior ammonia production rate, with the highest apparent quantum efficiency (AQE) of 1.24% at 808 nm and an appreciable solar-to-ammonia conversion efficiency (SACE) of 0.057% for NH3 production. Besides MoO3−x
, WO3−x
is also an excellent candidate for Schottky-barrier-free plasmonic photocatalysts. We have synthesized square WO3−x
nanoplates hydrothermally. The WO3-x
nanoplates possess regular morphology, wide light absorption, and great ammonia production performance. Furthermore, by introducing doping elements (Mo, V, Fe, Bi) into WO3−x
nanoplates, the N2 activation and dissociation can be improved by refining the defect states. The Mo-doped WO3−x
nanoplates exhibit the highest ammonia production rate of 145.50 μmol g−1 h−1 with SACE of 0.011% since the Mo species can promote the chemisorption and polarization of N2 molecules and significantly reduce the reaction barrier. Notably, Mo-doped WO3−x
nanoplates are a dual-function photocatalyst that can oxidize water to hydrogen peroxide in a stoichiometric ratio while synthesizing ammonia.

Figure 1