Armchair Janus WSSe Nanotube Designed with Selenium Vacancy as a Promising Photocatalyst for CO2 Reduction

Abstract

Photocatalytic conversion of carbon dioxide into chemical fuels offers a promising way to not only settle growing environmental problems but also provide a renewable energy source. In this study, through first-principles calculation, we found that the Se vacancy introduction can lead to the transition of physical-to-chemical CO₂ adsorption on Janus WSSe nanotube. Se vacancies work at the adsorption site, which significantly improves the amount of transferred electrons at the interface, resulting in the enhanced electron orbital hybridization between adsorbents and substrates, and promising the high activity and selectivity for carbon dioxide reduction reaction (CO₂RR). Under the condition of illumination, due to the adequate driving forces of photoexcited holes and electrons, oxygen generation reaction (OER) and CO₂RR can occur spontaneously on the S and Se sides of the defective WSSe nanotube, respectively. The CO₂ could be reduced into CH₄, meanwhile, the O₂ is produced by the water oxidation, which also provides the hydrogen and electron source for the CO₂RR. Our finding reveals a candidate photocatalyst for obtaining efficient photocatalytic CO₂ conversion.

Keywords: CO2 reduction; Janus WSSe nanotube; Se vacancy; photocatalysis.

Figure 1

(a) The adsorption sites (marked with red circles) in consideration of the pristine Janus WSSe nanotube. (b) The adsorption energy as well as the top (upper) and side (lower) views of the optimized configurations of CO₂ gas molecule adsorbing on pristine WSSe nanotube with different adsorption sites. The gray, red, orange, yellow, and blue balls each represent C, O, Se, S, and W atoms. (c) The enlarged view for the top view of center adsorption site optimized structure. The adsorption distance between the substrate and the adsorbate is represented by the dark blue, d. Transfer of charge from substrate to CO₂ molecule identified by the blue arrow.

Figure 2

(a) The total state density of the pristine CO₂ gas molecules and (b) the total state density of the pristine WSSe nanotube. (c) The partial state density of the adsorption system, where WSSe nanotube is shown in dark blue and CO₂ is shown in red. (d) Partial state densities of the C p orbitals (cyan) of the adsorbed CO₂ gas molecule and the Se p orbitals (black) of the Se atom most nearby the adsorbed CO₂ molecule. Fermi level is expressed by the vertical dashed line.

Figure 3

Top view (a) of the optimized structure and CDD (b) of Janus WSSe nanotube with Se vacancy adsorbed CO₂ gas molecules. Cyan (yellow) areas indicate charge depletion (accumulation). The isosurface level is 0.002 eÅ⁻³. Transfer of charge from substrate to CO₂ molecule identified by the blue arrow.

Figure 4

The total state density of WSSe nanotube with Se vacancy (a). The partial state density (b) of the adsorption system, the defective WSSe nanotubes are shown in dark blue, and CO₂ is shown in red. (c) Partial state densities of adsorbed CO₂ gas molecules in C p orbitals (cyan), O p orbitals (purple), and W d orbitals of two W atoms in substrate (orange). The vertical dashed line shows the Fermi level.

Figure 5

Band structures of (a) defective and (b) pristine Janus WSSe nanotubes. The gap states caused by the Se vacancy are marked with blue lines. The black number represents the value of band gap.

Figure 6

(a) Optical absorbance of pristine and defective Janus WSSe nanotubes. (b) Schematic diagram of band edge position of Janus WSSe nanotube relative to normal hydrogen electrode (NHE) at pH = 0. E_AVS represents the energy level relative to the absolute vacuum scale (AVS). The pink arrow represents the orientation of the built-in electric field.

Figure 7

The Gibbs free energy diagrams for the (a) 8 e pathway of CO₂RR and (b) 4 e pathway of OER on the defective Janus WSSe nanotube under different light conditions. The extra potentials provided by photogenerated electrons and holes are 0.73 and 2.77 V, respectively.

Figure 8

(a) Gibbs free energy diagram of HER on defective Janus WSSe nanotube. (b) ΔGco_2* (pink bar) vs. ΔG_H* (brown bar) of defective Janus WSSe nanotube. * means the adsorption site.