nanomaterials Article Effect of Morphology and Plasmonic on Au/ZnO Films for Efficient Photoelectrochemical Water Splitting Mohamed Zayed 1 , Nourhan Nasser 1 , Mohamed Shaban 1,2, * , Hind Alshaikh 3 , Hany Hamdy 1 and Ashour M. Ahmed 1   Citation: Zayed, M.; Nasser, N.; Shaban, M.; Alshaikh, H.; Hamdy, H.; Ahmed, A.M. Effect of Morphology and Plasmonic on Au/ZnO Films for Efficient Photoelectrochemical Water Splitting. Nanomaterials 2021, 11, 2338. https://doi.org/10.3390/ nano11092338 Academic Editor: Jacinto Sá Received: 1 August 2021 Accepted: 30 August 2021 Published: 8 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Nanophotonics and Applications (NPA) Laboratory, Physics Department, Faculty of Science, Beni-Suef University, Beni-Suef 62514, Egypt; m.zayed88ph@yahoo.com (M.Z.); Nnourhan673@gmail.com (N.N.); hshamdy@hotmail.com (H.H.); ashour.elshemey@gmail.com (A.M.A.) 2 Department of Physics, Faculty of Science, Islamic University in Madinah, Al-Madinah Al-Munawarah 42351, Saudi Arabia 3 Chemistry Department, Science and Arts College, Rabigh Campus, King Abdulaziz University, Jeddah 21911, Saudi Arabia; hfalshakh@kau.edu.sa * Correspondence: mssfadel@aucegypt.edu Abstract: To improve photoelectrochemical (PEC) water splitting, various ZnO nanostructures (nanorods (NRs), nanodiscs (NDs), NRs/NDs, and ZnO NRs decorated with gold nanoparticles) have been manufactured. The pure ZnO nanostructures have been synthesized using the successive ionic-layer adsorption and reaction (SILAR) combined with the chemical bath deposition (CBD) process at various deposition times. The structural, chemical composition, nanomorphological, and optical characteristics have been examined by various techniques. The SEM analysis shows that by varying the deposition time of CBD from 2 to 12 h, the morphology of ZnO nanostructures changed from NRs to NDs. All samples exhibit hexagonal phase wurtzite ZnO with polycrystalline nature and preferred orientation alongside (002). The crystallite size along (002) decreased from approximately 79 to 77 nm as deposition time increased from 2 to 12 h. The bandgap of ZnO NRs was tuned from 3.19 to 2.07 eV after optimizing the DC sputtering time of gold to 4 min. Via regulated time-dependent ZnO growth and Au sputtering time, the PEC performance of the nanostructures was optimized. Among the studied ZnO nanostructures, the highest photocurrent density (J ph ) was obtained for the 2 h ZnO NRs. As compared with ZnO NRs, the J ph (7.7 mA/cm 2 ) of 4 min Au/ZnO NRs is around 50 times greater. The maximum values of both IPCE and ABPE are 14.2% and 2.05% at 490 nm, which is closed to surface plasmon absorption for Au NPs. There are several essential approaches to improve PEC efficiency by including Au NPs into ZnO NRs, including increasing visible light absorption and minority carrier absorption, boosting photochemical stability, and accelerating electron transport from ZnO NRs to electrolyte carriers. Keywords: ZnO nanostructures; Au Surface plasmonic; photoelectrochemical; water splitting 1. Introduction Energy is one of the most pressing issues confronting humanity in the 21st century. Most of world’s energy (approximately 80%) originates from the burning of fossil fuels such as oil, coal, and natural gas. Unfortunately, these fossil fuels have many drawbacks [1]. Fossil fuels are non-renewable sources fuels, and within a limited period, they will in- evitably run out. The combustion of fossil fuels is followed by CO 2 emissions due to the reaction between carbon in fossil fuels and O 2 gas. This leads to a reduced amount of oxygen in the atmosphere and threatening life on earth [2]. The CO 2 gas emitted into the atmosphere often increases temperatures and triggers greenhouse effects and climate change [3]. It is, therefore, very important to use all the available renewable energy sources to address these problems [4,5]. Nanomaterials 2021, 11, 2338. https://doi.org/10.3390/nano11092338 https://www.mdpi.com/journal/nanomaterials