High-Efficiency Photocatalysts for CO2 Conversion Based on MoS2/CdS/TiO2 Nanotubes Heterostructures
Authors: Du Kang , Liu Guohua , Chen Xuyuan , Wang Kaiying 
Published: February 2020

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-Lα (2.29 eV), Mo-Lβ (2.40 eV), Cd-Lα (3.13 eV), Cd-Lβ (3.32 eV), and S-Kα (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).

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Figure 1