Hetero-Structured Palladium-Coated Zinc Oxide Photocatalysts for Sustainable Water Treatment

Abstract

Hetero-structured catalysts have recently attracted considerable interest from researchers in various fields, including electronics, sensing, energy, and photocatalysis. Treated water contains many harmful microorganisms and organic contaminants, which can be effectively removed through an advanced photocatalytic oxidation process. Photocatalysts with wide band gaps, such as semiconductor oxides like titanium dioxide (TiO 2 ) and zinc oxide (ZnO), are typically utilized to decompose these organic pollutants. These generate a charge when exposed to UV-A light irradiation and produce oxidation radicals that cause the decomposition of the organic matter present in water. Metals such as Pd, Ag, Au, Cu, and Ni are being studied as photocatalysts. Semiconducting oxides and metals with appropriate band gaps and Fermi levels can effectively capture the charge generated by the oxide owing to the Moss–Burstein effect. These reactions can enhance the photocatalytic effect by hindering the recombination of the generated holes and electrons, thereby increasing the probability of charge survival. In this study, the performance of zinc oxide nanowires (ZnO NWs) decorated with Pd nanoparticles was evaluated as photocatalysts for sustainable water treatment. To maximize the number of active sites on the surface of the ZnO nanowires, Pd nanoparticles were uniformly deposited via atomic layer deposition (ALD). In ALD, a very thin film that can be deposited in one cycle is created with less than one atomic layer owing to the precursor oxidation reaction. Additionally, the ALD process provides all the surfaces with sufficient time and chemical sources to react, resulting in a uniform amount of deposited material. This unique technology is suitable for the large-area deposition of atomic-level thickness surface materials. Therefore, ALD was used to deposit Pd onto the catalyst surface. The ZnO nanowires were initially fabricated in the form of seeds by ALD and grown via a hydrothermal method. Subsequently, ALD Pd nanoparticles with an average particle size of 3.75 nm were deposited on the enlarged ZnO nanowires. The prepared hetero-structured ALD Pd-deposited ZnO nanowires were analyzed using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and valence band X-ray photoelectron spectroscopy. A new Fermi level of the Pd-ZnO hetero-structure was formed, which was lower than the conduction band energy level of ZnO. The new Fermi level suppresses the recombination of the conduction band and valence band electron holes under UV-A irradiation. The enhanced charge separation enhances the photocatalytic activity. In addition, compared to ZnO NWs, ALD Pd-deposited ZnO NWs produced higher amounts of reactive oxygen species and improved the decomposition rate of organic pollutants in water. To evaluate the performance of the prepared photocatalyst, the decomposition rates of 4-chlorophenol (4-CP), 4-chlorobenzoic acid, and furfuryl alcohol were measured. Regarding the decomposition rate of 4-CP, the photocatalytic performance increased significantly from 42.5 % to 62.8 % when the ZnO nanowires were decorated with Pd nanoparticles. Figure 1