Controllable surface defect engineering on transition-metal trichalcogenide

NUS scientists have developed a method for controllable introduction of two different types of sulphur vacancies into zirconium trisulfide (ZrS₃) turning it into an efficient photocatalyst for hydrogen peroxide (H₂O₂) generation and benzylamine oxidation.

The introduction of defects can cause unexpected changes in the physical and chemical properties of materials. As a result, defect engineering has been a versatile tool for developing more efficient photocatalysts in chemical reactions. In photocatalytic applications, the introduction of defects can have a significant impact on the optical absorption, charge carrier dynamics, and surface catalysis kinetics of the materials. Better understanding of the structure-activity relationships brought about by the introduction of these defects can result in the development of more efficient photocatalytic materials.  

A research team led by Prof CHEN Wei from both the Departments of Physics and Chemistry, National University of Singapore has developed a method for controllable introduction of two different types of defects, the disulfide anions (S₂^2-) and the sulfide ion (S^2-) vacancies into ZrS₃ nanobelts (Figure (a) to (f)). The ZrS₃ nanobelts are long one-dimensional nanostructures that look like ribbons. The researchers found that the S₂^2- and S^2- vacancies can be introduced into the nanobelt material through two different methods (Figure (g) and (h)). For S₂^2- vacancies, this involves annealing the ZrS₃ nanobelt at 700℃ under vacuum conditions. For S^2- vacancies, a lithium-based hydrothermal method is used. By varying the annealing time (10, 15, and 20 mins) and amount of lithium present, defect engineered ZrS₃ material with varying amount of S₂^2- vacancies and S^2- vacancies can be obtained.

The researchers found that this defect engineered ZrS₃ material can enhance the photocatalytic production of H₂O₂ coupled with the selective oxidation of benzylamine to benzonitrile in water. They systematically investigated the effects of S₂^2- and S^2- vacancies on the charge carrier dynamics and photocatalytic performance. Their research findings show that the S₂^2- vacancies can significantly facilitate the separation of photogenerated charge carriers. Separately, the S^2- vacancies not only promote the electron conduction and hole extraction in the photocatalytic process but they also improve the kinetics of the benzylamine oxidation. These two different types of vacancies in the ZrS₃ material work together to improve the performance of the photocatalytic reaction. Under illumination by a simulated sunlight, the ZrS₃ material produces H₂O₂ and benzonitrile at a rate of 78.1 ± 1.5 and 32.0 ± 1.2 μmol h^-1 respectively.  

Prof Chen said, “Our research findings open up a new route for defect engineering and promise a potential strategy for the study of structure-activity relationships for the design and development of more efficient photocatalysts.”

Figure (a-f) shows the schematic process of the transformation of monoclinic zirconium trisulfide, ZrS₃ (ICCD PDF no. 30-1498) into hexagonal zirconium sulfide, ZrS₂ (ICCD PDF no. 11-0679) from the [010] (a-c) and [001] (d-f) views. Under heat treatment in vaccum conditions, ZrS₃ (a, d) releases sulphur ions to form a distorted crystal structure of ZrS₂ (b, e). The distorted crystal structure with the sulphur vacancies then undergoes structural relaxation by adjusting the length and angle of its bonds (c, f). Figure (g and h) shows the different type of sulphur vacancies. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of (g) ZrS₃ with S₂^2- vacancies and (h) ZrS₃ with both S₂^2- and S^2- vacancies measured from a spherical aberration-corrected transmission electron microscope (TEM). Inset: the crystal lattice of ZrS₃ along the [001] orientation. The red and yellow circles represent S₂^2-and S^2- vacancies, respectively. [Credit: Nature Communications]

Reference

Tian ZL; Han C*; Zhao Y; Dai WR; Lian X; Wang YN; Zheng Y; Shi Y; Pan X; Huang ZC; Li HX; Chen W*, “Efficient photocatalytic hydrogen peroxide generation coupled with selective benzylamine oxidation over defective ZrS₃ nanobelts” NATURE COMMUNICATIONS Volume: 12 Issue: 1 Article Number: 2039 DOI:10.1038/s41467-021-22394-8 Published: 2021.