Photoelectrochemical water splitting (PEC) is a viable solution for addressing future environmental and energy concerns. However, due to the high cost of photoelectrodes and the limitations of sluggish kinetics, practical applications are limited. To achieve that goal, effective solutions are still needed to create low-cost photoelectrodes suited for PEC. Transistion metal sulfides (TMS) materials have a high potential for use as photoelectrodes due to their favorable and impressive properties¹. The photoactivity of TMS is typically unlimited due to low reflection at the grain boundary. The individual atomic thin sheets with higher surface area compared to bulk materials allow for faster diffusion of photoinduced charge carriers primarily due to the less interface between their grains. Their lower interfacial contact resistance enables enhanced charge dynamics and, as a result, improved electrochemical reaction kinetics. When compared to other TMS, CdS has a direct bandgap and excellent photoactivity. However, its toxicity and photocorrosion make it unsuitable for long-term practical use. WS₂ is well-known layered transition metal sulfide that has a 2D molybdenite structure. It is non-toxic in acidic/neutral solutions that are chemically stable. It has been demonstrated to be more stable against oxidation and photo corrosion than CdS, which is extensively employed in photocatalysis². However, because of the recombination of photoinduced charge carriers, pure WS₂ exhibits relatively poor photocatalytic efficiency and stability. As a result, combining WS₂ based photoactive nanomaterials with other suitable materials has helped overcome this challenge. Transistion metal sulfides with acceptable band structures for heterostructure construction with WS₂ may enable breakthroughs in photocatalytic and photo-electrochemical processes.

The primary focus of this work is to construct heterostructures by combining CdS and WS₂ and to investigate their role as photoanodes of semiconductor materials in PEC water splitting. In this work, we have synthesised heterostructures using a facile solution-reaction method and Successive Ionic Layer Adsorption Reaction deposition (SILAR) technique which is intrinsically cost effective and scalable³. The heterostructure is characterised by XRD, SEM and TEM. We also studied the performance enhancement of this heterostructure using Linear sweep voltammetry and Electrochemical impedance spectroscopy. Pristine WS₂ and CdS showed photocurrent densities of 3 μA cm^-2 and 20 μA cm^-2. However, heterostructures formation led to much improved photocurrents. Photocurrent densities of 151 μA cm^-2 were observed for WS₂/CdS heterostructures respectively at 1.23 V vs RHE. The enhanced improvement in photoelectrochemical performance in the case of the heterostructures could be due to the following factors: (a) the intimate interaction of this unique heterostructure improves adhesion and lowers the photoinduced charge transfer barrier; (b) increased light harvesting capability; and (c) improved charge dynamics of photoinduced charge carriers due to proper band alignment. The current study paves the way for further research into the creation of metal sulfide heterostructures that generate analogous photoelectrode materials.

Keywords: Photoelectrochemical water splitting, metal sulfides, photoelectrodes.

References:

1. D. J. Late, C. S. Rout, D. Chakravarty and S. Ratha, Emerging Energy Applications of Two - Dimensional Layered Materials, Can. Chem. Trans., 2015, 3, 118–157, DOI:10.13179/canchemtrans.2015.03.02.0174.
2. X. Zong, J. Han, G. Ma, H. Yan, G. Wu and C. Li, Photocatalytic H₂ Evolution on CdS Loaded with WS₂ as Cocatalyst under Visible Light Irradiation, J. Phys. Chem., 2011, 12202–12208, DOI:10.1021/jp2006777.
3. M. Velpandian, S. Pulipaka, A. Tikoo, and P. Meduri, Improved charge carrier dynamics of WS₂ nanostructures by the way of CdS@WS₂ heterostructures for use in water splitting and water purification, Sustain. Energy Fuels, 2020, 4, 4096-4107, DOI:10.1039/d0se00533a.