Citation: Levinas, R.; Tsyntsaru, N.; Cesiulis, H.; Viter, R.; Grundsteins, K.; Tamašauskait˙ e-Tamaši ¯ unait ˙ e, L.; Norkus, E. Electrochemical Synthesis of a WO 3 /MoS x Heterostructured Bifunctional Catalyst for Efficient Overall Water Splitting. Coatings 2023, 13, 673. https://doi.org/ 10.3390/coatings13040673 Academic Editor: Ming-Tzer Lin Received: 28 February 2023 Revised: 21 March 2023 Accepted: 23 March 2023 Published: 25 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Article Electrochemical Synthesis of a WO 3 /MoS x Heterostructured Bifunctional Catalyst for Efficient Overall Water Splitting Ram ¯ unas Levinas 1,2, * , Natalia Tsyntsaru 1,3 , Henrikas Cesiulis 1 , Roman Viter 4,5 , Karlis Grundsteins 4 , Loreta Tamašauskait˙ e-Tamaši ¯ unait˙ e 2 and Eugenijus Norkus 2 1 Faculty of Chemistry and Geosciences, Vilnius University, 03225 Vilnius, Lithuania 2 State Research Institute, Center for Physical Sciences and Technology (FTMC), 10257 Vilnius, Lithuania 3 Institute of Applied Physics, Moldova State University, 2028 Chisinau, Moldova 4 Institute of Atomic Physics and Spectroscopy, University of Latvia, 1586 Riga, Latvia 5 Center for Collective Use of Scientific Equipment, Sumy State University, 40018 Sumy, Ukraine * Correspondence: ramunas.levinas@chf.vu.lt Abstract: Photo-/electrochemical water splitting can be a suitable method to produce “green” hy- drogen and oxygen by utilizing renewable energy or even direct sunlight. In order to carry out photoelectrochemical (PEC) water splitting, a photoanode based on transition metal oxides, which absorbs photons and produces photoexcited electron–hole pairs, is needed. The positively charged holes can then participate in the water oxidation reaction. Meanwhile, a cathodic hydrogen evolution reaction (HER) can occur more efficiently with electrocatalytic materials that enhance the adsorption of H + , such as MoS 2 . In this study, it was shown that WO 3 /MoS x heterostructured materials can be synthesized by an electrochemical method called plasma electrolytic oxidation (PEO). During this process, many micro-breakdowns of the oxide layer occur, causing ionization of the oxide and electrolyte. The ionized mixture then cools and solidifies, resulting in crystalline WO 3 with incorpo- rated MoS x . The surface and cross-sectional morphology were characterized by SEM-FIB, and the coatings could reach up to 3.48 μm thickness. Inclusion of MoS x was confirmed by EDX as well as XPS. Synthesis conditions were found to have an influence on the band gap, with the lowest value being 2.38 eV. Scanning electrochemical microscopy was used to map the local HER activity and correlate the activity hotspots to MoS x ’s content and surface topography. The bifunctional catalyst based on a WO 3 /MoS x heterostructure was evaluated for PEC and HER water-splitting activities. As a photoanode, it could reach up to 6% photon conversion efficiency. For HER in acidic media, a Tafel slope of 42.6 mV·dec −1 can be reached. Keywords: plasma electrolytic oxidation; tungsten oxide; molybdenum sulfide; heterostructure; water splitting; electrocatalysis; hydrogen evolution reaction; photoanode; scanning electrochemical microscopy 1. Introduction In the near future, in order to drive certain industrial and technological processes, utilization of solar energy will become more widespread than ever. This is proven by eco- nomic outlook reports such as that issued by the International Energy Agency, Renewables 2022, which forecasts global solar photovoltaic capacity to triple over the period 2022–2027. Furthermore, solar energy is actively being investigated as a viable solution for such issues as CO 2 reduction and water splitting to produce hydrogen [1,2]. Hydrogen in particular is a promising fuel for the future as it has the largest gravimetric current density of all known substances (~120 kJ·g −1 )[3], and therefore could be used to power energy-intensive technologies. Admittedly, the use of liquid hydrogen as an energy carrier faces many fundamental problems, one of which is its production. Nowadays, the production of hydrogen cannot be separated from the strive for climate neutrality. Although hydrogen can be produced in large quantities by steam methane Coatings 2023, 13, 673. https://doi.org/10.3390/coatings13040673 https://www.mdpi.com/journal/coatings