CO2 reduction to storable fuels or valuable chemical products provides a carbon-neutral cycle that can mitigate the rapid consumption of fossil fuels and increasing CO2 emissions. Although solar-driven CO2 reduction holds great promise for sustainable energy, the role and control of atomic-level active sites in governing intermediate formation and conversion pathways remains poorly understood.
Here, NTU researchers demonstrate a capped VLS growth method employed to grow continuous ultrathin alloy films of molybdenum vanadium sulfide with sulfur vacancies, i.e., Svac-Mo1-xVxS2. The findings indicate a correlation between the density of Svac and the vanadium concentration, leading to the formation of V–Svac pairs, which serve as the active sites for CO2 reduction under solar light with water.
This research presents a practical method for incorporating vanadium into atomically thin hexagonal MoS2, with the objective of enhancing its optoelectronic properties. The primary motivation behind this study is to develop more efficient photocatalysts for the reduction of CO2 in the gas phase, addressing the critical issue of rising atmospheric CO2 levels.
To achieve this, the authors employed a capped, or membrane-controlled, vapor–liquid–solid (VLS) growth technique. This approach enables the production of wafer-scale, continuous ultrathin Mo1-xVxS2 alloy films that feature sulfur vacancies (Svac). Compared to previous methods, this technique significantly enhances vanadium incorporation, thus addressing a major limitation in controlling the alloying process.
The research, published in ACS Nano ("Capped Vapor–Liquid–Solid Growth of Vanadium-Substituted Molybdenum Disulfide Ultrathin Films for Enhanced Photocatalytic Activity"), involved careful optimization of several growth parameters, such as the thickness of the SiO2 capping layer, growth temperature, and the dimensions of the solid precursors confined within a nanometer-scale environment. The SiO2 membrane plays crucial roles throughout the growth process: it prevents the escape of vanadium-containing species, supplies sulfur and hydrogen to the molten phase via gas diffusion, and promotes uniform, symmetric two-dimensional grain growth across the substrate.
Microscopy analyses and theoretical calculations indicate a strong correlation between the presence of vanadium atoms and the density of sulfur vacancies, with increasing vanadium content resulting in a higher vacancy density. This pairing of vanadium and sulfur vacancies leads to charge transfer from vanadium to adjacent sulfur and molybdenum atoms, resulting in pronounced photoluminescence quenching and modifications to the electronic states.
As a photocatalyst, bilayer Svac–Mo1-xVxS2 achieved a CO2-to-CO conversion rate approximately five times greater than that of pristine MoS2 when exposed to simulated sunlight. The internal quantum efficiency (IQE) reached around 0.017%, with the catalyst demonstrating stability for at least 20 hours. The enhanced catalytic activity is largely attributed to the formation of V–Svac active sites, which improve light absorption in the visible spectrum, alter the band structure, and facilitate charge transfer and transport. To see article on NanoWerk: https://www.nanowerk.com/nanotechnology-news3/newsid=68487.php
To see article on NanoWerk: https://www.nanowerk.com/nanotechnology-news3/newsid=68487.php