Among semiconductors, cuprous oxide (Cu₂O) is one of the most investigated in the framework of photoelectrochemical water splitting mainly because it has a bandgap of about 2.2 eV, high enough to overcome the thermodynamic requirements for water electrolysis but relatively narrow, allowing to absorb a significant amount of the solar irradiation. However, a major issue is found upon irradiation, where the promoted electrons are more likely to reduce the oxide film to the metallic state instead of reducing protons to molecular hydrogen, reducing its own photoactivity in a short amount of time [1]. The strategies to overcome this issue consist mainly in the deposition of protective layers, improving also the charge separation and transfer [2]. The formation of Cu₂O/CuO heterojunction, simply achieved by controlled oxidation of the cuprous film in air, has been already investigated and showed good improvement in the electrode stability [3], further improved with the implementation of a catalyst e.g. Ni, Pt [4,5]. In the view of a scalable process with low cost, the implementation with earth abundant material is required and most important the fabrication route has to be feasible for large area production.

Based on this idea, the modification of Cu₂O/CuO heterostructure through sulfurization is proposed as a novel synthesis for CuS catalyst, achieved by conversion of the underneath cupric oxide (CuO), resulting in the formation of Cu₂O/CuO/CuS heterostructure, already showed to be promising [5]. Oxidation of electrodeposited Cu₂O/FTO was carried out in open air in a range of temperatures from 400-500 °C for 0.5-2 h. Subsequently, CuS catalyst is obtained by sulfurization in a nitrogen atmosphere containing elemental sulfur heated at 300-450 °C for 1-5 minutes. Scanning electron microscopy, X-Ray diffraction, and X-Ray photoelectron spectroscopy were carried out for morphological and compositional characterization. Photoactivity was investigated in a conventional three-electrochemical cell using a solar simulator (100 mW/cm², AM 1.5 G) as a light source. The photoelectrochemical results are described in the view of fabrication of a tandem cell where the Cu2O/CuO based photocathode is coupled with BiVO4 based photoanode.

References

[1] Wu, Lingling, et al. "Photoelectrochemical stability of electrodeposited Cu2O films." The Journal of Physical Chemistry C 114.26 (2010): 11551-11556.

[2] Paracchino, Adriana, et al. "Highly active oxide photocathode for photoelectrochemical water reduction." Nature materials 10.6 (2011): 456.

[3] Yang, Yang, et al. "Cu 2 O/CuO bilayered composite as a high-efficiency photocathode for photoelectrochemical hydrogen evolution reaction." Scientific reports 6 (2016): 35158.

[4] Dubale, Amare Aregahegn, et al. "Heterostructured Cu 2 O/CuO decorated with nickel as a highly efficient photocathode for photoelectrochemical water reduction." Journal of Materials Chemistry A 3.23 (2015): 12482-12499.

[5] Dubale, Amare Aregahegn, et al. "A highly stable CuS and CuS–Pt modified Cu 2 O/CuO heterostructure as an efficient photocathode for the hydrogen evolution reaction." Journal of Materials Chemistry A 4.6 (2016): 2205-2216.