Currently, environmental pollution and energy demanding are two urgent challenges for humanity [1]. It is believe that CO₂ emission is the primary greenhouse gas causing global warming. In theory, CO₂ could be a potential feedstock for the production of fuel, energy, and value-added chemicals while reducing greenhouse gases [2]. Since Halmann and Inoue et al. first reported the photocatalytic reduction of CO₂ in the late 1970s, scientists have attempted to look for practically feasible, inexpensive, environmentally friendly, and energy efficient approaches for the photocatalytic conversion of CO₂ [3, 4]. Although TiO₂ is the most popular photocatalyst for CO₂ conversion based on its robust reactivity, commercial availability and chemical stability, it only absorbs ultraviolet light (~4% of solar spectrum) and thus significantly limits its application [5]. Therefore, the development on high-efficiency visible-light-driven photocatalysts have become a hot issue in the photocatalysis field. Metal sulfide nanomaterials have attracted attention because of their excellent properties, earth-abundant, and promising applications in energy conversion and storage. Using metal sulfides to synthesize heterostructural photocatalysts might be a breakthrough for CO₂ conversion.

In this work, TiO₂ nanotubes (TNT) photoelectrodes were fabricated by anodization [6]. Subsequently, MoS₂ and CdS nanoparticles were deposited on TNT by magnetron sputter technique. The catalytic ability of as-formed heterostructure photocatalysts were investigated through photocatalytic CO₂ conversion. Fig. 1 shows the schematic diagram and physical buildup of CO₂ conversion measurement system. The CO₂ conversion measurements were conducted in a gastight reactor (~25 mL) with a quartz plate at the top, inlet, outlet and sampling port at the bottom, shown in Fig. 1 (d). A 500 W Xenon lamp was used as the illuminating source. Fig. 2 (a) the surface morphologies of pristine TNT, indicating the thickness of tube wall is ~16.9 nm. After deposited with MoS₂ and CdS nanoparticles, the thickness of tube wall are increased to ~45.1 nm and ~40.7 nm, shown in Fig. 2 (b) and (c). Thus, the size of MoS₂ and CdS nanoparticlesis estimated as ~14 nm and ~12 nm. In addition, energy-dispersive X-ray spectroscopy (EDX) spectra of samples show several new peaks which correspond to Mo-L_α (2.29 eV), Mo-L_β (2.40 eV), Cd-L_α (3.13 eV), Cd-L_β (3.32 eV), and S-K_α (2.31 eV), shown in Fig. 2 (d). Fig. 3 (a) shows the UV-Vis absorption spectra of TNT, TNT-MoS₂, TNT-CdS, and TNT-MoS₂-CdS. Compared with pristine TNT, TNT-MoS₂, TNT-CdS, and TNT-MoS₂-CdS exhibit the enhancements in different degrees at the visible regime from 360 nm to 800 nm. Fig. 3 (b) shows the bandgap determination using Kubelka-Munk function. The bandgap of TNT-MoS₂, TNT-CdS, and TNT-MoS₂-CdS are 2.58 eV, 2.92 eV, and 2.77 eV, which are narrower than that of pristine TNT (3.08 eV). Fig. 3 (c) and (d) show the gases yield by pristine TNT and TNT-MoS₂-CdS as a function of illumination time. The yield of H₂, CO, and CH₄ by pristine TNT are 0.55 µmol/cm², 0.37 µmol/cm², and 0.77 µmol/cm² after illumination for 5 hours. Excitedly, TNT-MoS₂-CdS shows the enhanced photocatalytic activity on gas yields, 53.44 µmol/cm² of H₂, 1.38 µmol/cm² of CO, and 11.53 µmol/cm² of CH₄. The specific photocatalytic activities of TNT-MoS₂-CdS heterojunction photocatlyst are 97 times, 3.8 times, and 15 times larger than that of pristine TNT for the yields of H₂, CO, and CH₄.

In conclusion, metal sulfide nanomaterials (MoS₂ and CdS) were deposited on anodized TiO₂ nanotubes through magnetron sputter technique. The synthesized TNT-MoS₂-CdS heterojunction nanocomposites manifested a superior performance for photocatalytic CO₂ conversion. This work may has the potential to provide new insights into the development of visible-light-driven nanocomposites as highly efficient photocatalysts for converting CO₂ into fuels.

Acknowledgments: The author Kang Du would like to acknowledge financial support from KD program at University College of Southeast Norway, and Norwegian Research Council-FRINATEK programme (231416/F20).

References

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