Boosted Visible-Light Photocatalysis of MoS2/g-C3N4 Composites by Atmosphere-Controlled Mo Species Evolution

To improve the visible-light-driven photocatalytic degradation efficiency of g-C3N4-based photocatalysts toward organic pollutants, a MoS2/g-C3N4 composite precursor was employed in this work, and the phase composition and defect environment of Mo species were regulated by post-annealing under air and N2 atmospheres, respectively, thereby constructing Mo-based/g-C3N4 (MCN) composites with distinct structural evolution characteristics. The results showed that the photocatalytic activity of the as-sonicated MCN composite toward methylene blue (MB) was only moderately improved, among which the 15% loading sample exhibited the best performance with a degradation efficiency of about 42.0% within 60 min. In contrast, annealing at 400 °C under N2 resulted in only a slight activity change, whereas the sample treated at 400 °C in air (Air-15% MCN) achieved an MB degradation efficiency of 99.9% within 60 min, together with a much higher pseudo-first-order reaction rate constant than that of the air-treated sample at a lower temperature. XRD, FT-IR and XPS analyses revealed that air annealing induced the conversion of MoS2 into highly crystalline MoO3 (or MoO3−x), leading to the formation of a reconstructed MoO3−x/g-C3N4 composite interface. Meanwhile, the increased high-binding-energy component in the O 1s spectrum and the EPR signal around g ≈ 2.00 further suggested the presence of more abundant defect-related centers in the air-treated sample. Although Air-15% MCN possessed a lower specific surface area than the untreated and N2-treated samples, it displayed enhanced visible-light absorption, higher transient photocurrent response, lower interfacial charge-transfer resistance, and accelerated carrier dynamics, indicating that the activity enhancement mainly originated from atmosphere-induced phase transformation, interfacial reconstruction, defect-related active centers, and improved charge separation/transfer, rather than from the surface area effect. Based on the above results, a possible interfacial charge-transfer pathway is tentatively proposed for the g-C3N4/MoO3−x interface formed after air treatment, which contributes to the efficient utilization of photogenerated carriers and the rapid degradation of MB. This work demonstrates that atmosphere-induced phase transformation is a simple and effective strategy for regulating the structure and photocatalytic performance of Mo-based/g-C3N4 composites, and provides useful guidance for the design of efficient visible-light photocatalysts.