Unraveling charge transfer pathways and mechanisms in CdS@CoWO₄ Z-scheme heterojunction photocatalysts for high-efficiency environmental remediation

Recently, direct Z-scheme heterojunction catalysts have become an important hotspot in photocatalytic applications due to their much stronger redox capabilities. Novel direct Z-scheme CdS@CoWO₄ spherical heterojunction photocatalysts were ingeniously designed and synthesized herein. The structural characterization and morphology analysis demonstrated that homogeneous heterojunctions between CdS and CoWO₄ were indeed formed in the composite photocatalysts. Moreover, CdS@CoWO₄ heterojunction photocatalysts exhibited excellent degradation capability towards refractory micropollutants (Pharmaceutical intermediates, dyes, and industrial additives, etc) in wastewater. Especially, the strongest photocatalytic activity reflected by the highest reaction kinetic parameter (k = 8.13 × 10⁻² min⁻¹) was observed in the CdS-2CoWO₄ catalyst with the mass ratio of CdS:CoWO₄ = 1:2, which is nearly 4 and 58 times as high as that of pure CdS (k = 2.21 × 10⁻² min⁻¹) and CoWO₄ (k = 1.39 × 10⁻³ min⁻¹), respectively. First-principles study combined with the radical species trapping experiments revealed that the charge transfer pathways and mechanisms in these heterojunctions favor the direct Z-scheme rather than the traditional Type-II mode. A key built-in electric field formed at the contact interfaces of CdS and CoWO₄ drives the Z-scheme charge transfer mechanism, which strengthens the redox capacity of photogenerated carriers and ultimately greatly enhances the photocatalytic activity of the heterojunction catalysts. It is worth applauding that CdS and CoWO₄ can form such Z-scheme heterostructures in a wide span of component ratios, which is of great benefit for their practical applications in environmental remediation.