(Digital Presentation) 2D Molybdenum Disulfide Nanosheets As Cocatalyst and Passivation Layer over CuInS2 Based Photocathode for Solar Hydrogen Evolution
Authors: Kumar Mohit , Subrahmanyam Challapali 
Published: September 2023

An excellent strategy to improve photoelectrochemical (PEC) water splitting is to develop cost-effective co-catalysts that do not use noble metals such as Platinum (Pt), Iridium (Ir), Ruthenium (Ru). Transition metal cocatalysts have shown excellent catalytic property towards hydrogen evolution (HER) and oxygen evolution reaction (OER). The surface active sites and lower free energy of H* adsorption (GH adsorption) on the surface are the factors ameliorating and boosting their catalytic activity. Molybdenum Disulfide (MoS2), one the transition metal dichalcogenide with 2D-sheet like structure with high surface area, active sites, and excellent property to catalyze HER comparable to Pt. The property of higher stability towards the photocorrosion as compared to other cocatalysts provides MoS2 with an additional feature of passivation layer. The sheet-like structure also enhances the light absorption minimizing the effect of parasitic light absorption.
Copper based oxides and chalcogenides are well known p-type semiconductors being investigated for photocathode materials to be used in unassisted PEC water splitting cell. CuInS2, a ternary copper chalcogenide material with tunable bandgap (Eg = 1.5-1.8 eV), high absorption coefficient (> 104), suitable band positions for hydrogen evolution reaction. Despite all the necessary features and properties, it suffers with the problem of high rate of recombination between photogenerated charge carriers, lower diffusional lengths and poor photostability. Several approaches have been developed to improve the PEC performance of CuInS2 that include decoration of metal NPs, heterojunction fabrication, deposition of electron (ETLs) and hole transporting layers (HTLs). CdS forms a proper heterojunction with CuInS2 owing to its conduction (CB) and valence (VB) offsets.
The integration of CuInS2 with CdS could form a facile heterojunction for the effective charge separation and reduce the rate of recombination and decoration of MoS2 over the CuInS2/CdS can be beneficial in terms of improved reaction kinetics at the surface and photostability by avoiding direct contact of CuInS2 and CdS with electrolyte. A CuInS2 nanosheet array-based photocathode that is modified with CdS and the co-catalyst MoS2. This green approach enhances water splitting under solar irradiation. By introducing CdS and MoS2, we significantly improved the visible light absorption of the modified hybrid photocathode (CIS/CdS/MoS2). Photoluminescence, impedance spectroscopy, and Mott–Schottky analysis confirmed that excited electron-hole pairs were better separated, the resistance of charge transfer was minimized, and the excited-state charge carrier concentration increased, leading to increased photocurrent.
Typical results demonstrated that CIS/CdS/MoS2 photocathode delivered higher photocurrent (-1.75 mA/cm2 at 0 VRHE) and HC-STH conversion efficiency (0.42% at 0.49 VRHE) than those of CIS and CIS/CdS photoelectrodes. We attribute this improved PEC performance to the synergetic impact of CdS in charge generation and transfer and MoS2 as a cocatalyst with active surface sites for proton reduction. The hydrogen evolution experiment showed CIS/CdS/MoS2 (56.9 μmol/h) showed high activity and rate of hydrogen evolution compared to CIS (22.6 μmol/h). This study not only demonstrates the promising nature of CuInS2-based light absorber photocathodes for solar energy utilization but also recommends the use of MoS2 as a cocatalyst for the proton reduction reactions for widespread applications in solar-to-hydrogen conversion.

Reference

(1) Kumar, M.; Meena, B.; Subramanyam, P.; Ummethala, G.; Malladi, S. R. K.; Dutta-Gupta, S.; Subrahmanyam, C. CuInS 2 Nanosheet Arrays with a MoS 2 Heterojunction as a Photocathode for PEC Water Splitting. Energy & Fuels
2023, 37 (3), 2340–2349. https://doi.org/10.1021/acs.energyfuels.2c03502.

Figure 1