MSc. Tahir Wahab

The design of type-II van der Waals (vdW) heterostructures is regarded as a promising route to produce green hydrogen via photocatalytic water splitting. To this aim, we propose novel vertically stacked vdW heterostructures based on ZnO and Janus VXY (X = Br, Cl, Y = Se, and Te) phases, and investigate their optoelectronic properties and photocatalytic performance by means of density functional theory simulations. The thermal stability of the heterostructures is confirmed by ab initio molecular dynamics simulations at 300 K. The HSE06 calculated band structures show that a specific stacking of ZnO-VBrSe and ZnO-VClSe exhibits an indirect band gap with type-II band alignment, while all other stackings exhibit a direct band gap with type-I band alignment. The type-II band alignment, along with the difference in the work function and the electrostatic potential between the ZnO and VXY monolayer, will result in a built-in electric field direct from the ZnO monolayer to the VXY monolayer which is crucial for photogenerated charge separation, and prevents the charge recombinations. The optical absorption coefficient alpha of all the considered ZnO-VXY heterostructures displays the first excitonic peak in the energy range required for photocatalysis applications. Based on the band edge potential analysis, all the studied systems are capable of starting an oxygen evolution reaction spontaneously, while some external stimuli will be required to initiate the hydrogen evolution reaction. The reported results suggest that the proposed ZnO-VXY vdW heterostructures have great potential for photocatalysis and optoelectronic device applications.

Two-dimensional (2D) materials have received enormous attention as photocatalysts for hydrogen production to address the worldwide energy crisis. In this study, we employed first-principles computations to systematically investigate the structural, opto-electronic, and photocatalytic properties of novel GeC-MX2 (M = Mo, W, X = S, Se) van der Waals (vdW) heterostructures for photocatalysis applications. Our results reveal that the GeC-MX2 heterostructures can absorb visible light. The type-II band alignment in GeC-MoS2 and GeC-WS2 enables the photogenerated electron-hole pairs to be separated continuously. The electron transfer from the GeC monolayer to MX2 monolayer leads to a large built-in electric field at the interface. This induced electric field is essential for preventing the recombination of photogenerated charges. Moreover, the band-edge locations suggest that GeC-MX2 heterostructures can be utilized as a photocatalyst for water splitting. Finally, the opto-electronic properties of these novel GeC-MX2 heterostructures facilitate their practical utilization in future photocatalysis applications.