Citation: Bakranova, D.; Nagel, D. ZnO for Photoelectrochemical Hydrogen Generation. Clean Technol. 2023, 5, 1248–1268. https://doi.org/ 10.3390/cleantechnol5040063 Academic Editor: Stéphane Grieu Received: 8 July 2023 Revised: 25 September 2023 Accepted: 7 October 2023 Published: 20 October 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/). clean technologies Review ZnO for Photoelectrochemical Hydrogen Generation Dina Bakranova 1,2, * and David Nagel 3, * 1 School of Natural and Social Sciences, Kazakh-British Technical University, Almaty 050000, Kazakhstan 2 Research Group altAir Nanolab, Almaty 050000, Kazakhstan 3 Department of Electrical Engineering, School of Engineering and Applied Science, George Washington University, Washington, DC 200052, USA * Correspondence: dinabakranova@gmail.com (D.B.); nagel@gwu.edu (D.N.) Abstract: The rise in the Earth’s surface temperature on an annual basis has stimulated scientific and engineering interest in developing and implementing alternative energy sources. Besides cost, the main requirements for alternative energy sources are renewability and environmental friendliness. A prominent representative that allows the production of “green” energy is the conversion of solar photons into a practical energy source. Among the existing approaches in solar energy conversion, the process of photoelectrochemical (PEC) hydrogen extraction from water, which mimics natural photosynthesis, is promising. However, direct decomposition of water by sunlight is practically impossible since water is transparent to light waves longer than 190 nm. Therefore, applying a photoelectrochemical process using semiconductor materials and organic compounds is necessary. Semiconductor materials possessing appropriately positioned valence and conduction bands are vital constituents of photoelectrodes. Certain materials exhibit semiconductor characteristics that facilitate the reduction-oxidation (RedOx) reaction of water (H 2 O) under specific circumstances. ZnO holds a unique position in the field of photocatalysis due to its outstanding characteristics, including remarkable electron mobility, high thermal conductivity, transparency, and more. This article offers an overview of studies exploring ZnO’s role as a photocatalyst in the generation of hydrogen from water. Keywords: ZnO; photocatalysis; plasmonic; nanocomposites; thin films 1. Introduction The deterioration of the ecology and climate of the planet, as well as the greenhouse effects caused by the excessive consumption of hydrocarbon energy carriers, motivate the scientific community to create alternative ways for energy production. One of the most promising prospects of alternative energy is the utilization and conversion of the Sun’s electromagnetic waves. Solar energy hits the Earth’s surface in a large amount, about 32 × 10 24 J per year. The conversion of light into the necessary form of energy is of great scientific and applied importance for humankind’s industrial and economic activities. The effective conversion of only 0.001% of the light energy coming from the Sun will satisfy the energy needs of modern society [1]. Two main methods of transforming solar energy into electricity or heat, are solar panels that generate electricity and solar collectors that concentrate thermal energy. The main problem from these methods is the storage and transportation of the resulting power. Therefore, the issue of creating alternative energy technologies is open [2]. One of the promising ways of solar energy utilization is the conversion of photons into chemical energy in the form of hydrogen gas [3]. Hence, there is a large literature on PEC, both books [4,5] and review articles [6,7], as well as individual articles. Today, fossil fuels, especially natural gas, are sources of about 95% of the hydrogen consumed in the world. However, fossil resources are not feasible for hydrogen resources because replenishment takes geological times, and much worse, the additional carbon Clean Technol. 2023, 5, 1248–1268. https://doi.org/10.3390/cleantechnol5040063 https://www.mdpi.com/journal/cleantechnol