Cobalt Oxide–Zinc Sulfide Junction Boosts Sacrificial-Agent-Free Photocatalytic Ammonia Synthesis

Chemists have reported a cobalt oxide–zinc sulfide heterostructure that converts nitrogen and water into ammonia using only light, with no externally added sacrificial agent. The optimized Co₃O₄/ZnS composite, prepared by in situ deposition, produced ammonia at a rate of 607.6 ± 9.1 µg·h⁻¹·gcat⁻¹ (n = 3), far exceeding the rates measured for the individual components: 131.2 ± 2.0 µg·h⁻¹·gcat⁻¹ for Co₃O₄ alone and 75.7 ± 1.9 µg·h⁻¹·gcat⁻¹ for ZnS alone.

The team characterized the materials with XRD, XPS, SEM, TEM, HRTEM, EDS and ICP analyses, all of which indicated the coexistence of Co₃O₄ and ZnS phases in intimate interfacial contact. Quantitative EDS analysis gave a Zn/S atomic ratio of 0.98 ± 0.04, consistent with the intended ZnS phase, and the actual ZnS loading in the optimized 2:1 sample was approximately 31.9 wt%. To confirm that the detected ammonia truly derived from the supplied nitrogen gas rather than contamination, the researchers performed ¹⁵N₂ isotopic labeling followed by ¹H NMR measurements, which supported the nitrogen source being the fed N₂.

Control experiments helped narrow down why the 2:1 composition performed best. BET surface area analysis, N₂ temperature-programmed desorption, and a Na₂S-treated Co₃O₄ control collectively indicated that the superior activity could not be explained solely by surface area, ZnS content, or sulfur treatment. Instead, the authors attribute the performance to the combined effects of an appropriate composition, N₂ adsorption, Co-based surface sites, and heterointerfacial interactions between the two phases.

On the mechanistic side, UV-vis diffuse reflectance spectroscopy, Mott-Schottky analysis and valence-band XPS established an experimental basis for the band alignment of the two semiconductors. Together with photocurrent and electrochemical impedance spectroscopy results, these measurements support a plausible S-scheme-like charge-transfer model, in which the n-type ZnS and p-type Co₃O₄ form a p–n heterojunction that promotes interfacial charge separation. The authors are careful to frame this as a proposed model: multi-frequency Mott–Schottky measurements and EPR spin-trapping experiments were consistent with the S-scheme-like pathway and p–n-junction-assisted carrier separation, but did not uniquely prove the microscopic interfacial transfer route.

The study also quantified the oxidative half-reaction. Water oxidation yielded 3.80 ± 0.18 µmol of O₂, and relative to the charge demand associated with 5.35 µmol of NH₃, the electron/hole balance was approximately 94.7%, supporting water oxidation as the major hole-consumption pathway. Durability was similarly encouraging: after five consecutive reaction cycles, 98.4% of the initial activity was retained, and post-reaction XPS, SEM and TEM showed that the principal surface chemical states, morphology and interfacial structure remained largely preserved.

Subject of Research: Sacrificial-agent-free photocatalytic nitrogen reduction to ammonia over a Co₃O₄/ZnS heterojunction

Article Title: Enhanced Sacrificial-Agent-Free Photocatalytic N₂ Reduction over a Co₃O₄/ZnS Heterojunction

Article References: Fu, S., Wang, X., Liu, Z., Hao, J., Li, W., Chen, X., Li, J., & Zhang, Q. (2026). Enhanced Sacrificial-Agent-Free Photocatalytic N₂ Reduction over a Co₃O₄/ZnS Heterojunction. Catalysis Letters, 156(11), Article 311. https://doi.org/10.1007/s10562-026-05556-5

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05556-5

Keywords: photocatalysis, nitrogen reduction, ammonia synthesis, Co3O4/ZnS heterojunction, S-scheme charge transfer, water oxidation, 15N2 isotope labeling, p-n heterojunction, solar fuels, Catalysis Letters, Enhanced, Sacrificial-Agent-Free