Marigold-flower-like CaIn2S4/TiO2 nanoheterostructures with dual vacancy defects enabling S-scheme charge transfer for superior photoelectrochemical and gas-sensing performance

The rational integration of CaIn 2 S 4 (CIS) nanosheets with TiO 2 nanorods was achieved via a facile hydrothermal method followed by calcination. This strategy enabled the fabrication of marigold flower-like CIS/TiO 2 nanoheterostructures enriched with dual vacancy defects, namely sulfur vacancies (V S ) and oxygen vacancies (V O ), under various CIS deposition conditions. Compared with the single-phase nanostructures of TiO 2 and CIS, the CIS/TiO 2 binary heterojunctions exhibited significantly enhanced photoelectrochemical and gas-sensing performances. These improvements are a direct reflection of the synergistic effect between the efficient V O mediated S-scheme charge transfer pathway and the large number of V S defects， together with abundant V S defects that provide favorable oxidation and reduction sites. In addition, this study proposes a self-consistent S-scheme interfacial charge transfer mechanism based on the results of femtosecond transient absorption and nanosecond time-resolved photoluminescence spectroscopy. Following this design principle, the optimized CIS/TiO 2 -60 nanocomposite achieved a photodegradation efficiency of 98.8% for methyl orange under UV–visible irradiation, representing enhancement factors of 6.3 and 4.6 relative to pristine TiO 2 and pure CIS, respectively. Simultaneously, the CIS/TiO 2 -60 heterostructure demonstrated remarkable NO 2 sensing performance at room temperature under UV–visible activation, with a sensitivity of 3.51-approximately 3.3, 1.5, 1.3, and 1.2 times higher than those of pristine TiO 2 , CIS/TiO 2 -20, CIS/TiO 2 -40, and CIS/TiO 2 -80, respectively. In addition, CIS/TiO 2 -60 exhibited the fastest response/recovery times (36.5 s / 49.3 s), outperforming pristine TiO 2 (51.5 s / 61.9 s). These results highlight that the synergistic effect arising from dual vacancy defects and the S-scheme CT process within the CIS/TiO 2 nanoheterojunction provides valuable insights and guidance for the design of high-performance materials for photocatalysis and gas-sensing.