PdO nanoparticles decorated TiO 2 film with enhanced photocatalytic and self-cleaning properties S. Veziroglu 1 , J. Hwang 1 , J. Drewes, I. Barg, J. Shondo, T. Strunskus, O. Polonskyi, F. Faupel * , O.C. Aktas * Chair for Multicomponent Materials, Institute for Materials Science, Faculty of Engineering, Kiel University, Kaiserstr. 2, Kiel, 24143, Germany article info Article history: Received 1 November 2019 Received in revised form 4 February 2020 Accepted 4 February 2020 Available online xxx Keywords: Thin film Photocatalysis Palladium Titanium oxide Gas aggregation source abstract Magnetron sputtering and gas aggregation source (GAS) approaches were combined for the preparation of columnar TiO 2 structures decorated with PdO nanoparticles (NPs). The totally solvent-free synthesis approach provides good control of surface coverage, size, morphology, and stoichiometry of PdO NPs in comparison to wet chemical synthesis methods. X-ray photoelectron spectroscopy (XPS) analysis showed that the heat treatment led to the formation of a mixed oxide state PdO/PdO 2 on the TiO 2 layer. A steady equilibrium between PdO (oxidation by free and adsorbed OH) and PdO 2 (reduced by trapped photo- generated electrons) phases under UV irradiation seems to provide an efficient electron-hole pair sep- aration. Such robust PdOeTiO 2 thin films have a strong potential for use as photocatalytic and self- cleaning windows or similar out-door technical surfaces. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Titanium (IV) oxide (TiO 2 ) has been used in diverse products, including paints, coatings, glazes, paper, inks, fibers, foods, phar- maceuticals, and cosmetics [1e4]. It is also the most extensively used material for photocatalytic and photovoltaic applications owing to its high conversion efficiency, stability, chemical inert- ness, and low cost [5,6]. Today TiO 2 based photodegradation is accepted as one of the most effective technologies for environ- mental remediation [7]. Especially the use of colloidal TiO 2 particles for the treatment of various kinds of wastes have been shown by various studies, as well as technology applications [4,8]. On the other hand, in some specific applications, such as water treatment, air, and odor cleaning, the use of TiO 2 in the thin-film form (rather than colloid form) is preferable because of its continuous nature and advantage in the catalyst regeneration process. Besides various advantages of TiO 2 , its fast recombination rate (of photogenerated electronehole pair) and large bandgap limits its use as an effective photocatalyst [9e12]. For instance, in the case of using TiO 2 in the thin-film form, the limited surface area (outperformed by their colloidal counterparts) becomes an addi- tional restriction [13]. Recently we proposed a simple approach to enhance the surface area of TiO 2 thin films by inducing nanocrack networks in sputter-deposited films [14]. On the other hand, charge separation in such TiO 2 thin films should also be improved to foster their use in photocatalytic applications. Various approaches, for instance, doping, metal loading, and/or introducing a heterojunction, have been proposed to efficiently separate the photogenerated electronehole pairs in TiO 2 [15e17]. Among all proposed methods, creating a heterojunction (the interface between two different semiconductors with unequal band structure can result in band alignment) by incorporating a second oxide photocatalyst with TiO 2 has been proved to be one of the most promising ways for achieving a high photocatalytic per- formance because of its feasibility and effectiveness for the spatial separation of electronehole pair [18e20] For instance, recently we demonstrated that TiO 2 eCeO 2 hybrid nanostructures exhibited excellent photocatalytic activity in comparison to bare TiO 2 and CeO 2 structures used separately [21]. A number of studies have been carried out on the incorporation of metal oxides, such as CeO 2 , WO 3 , Fe 2 O 3 , ZnO, and PdO with TiO 2 to enhance the photocatalytic activity [22e24]. In recent years, Pd and PdO catalysts have been extensively studied, and they are considered as effective catalysts, particularly for CO oxidation and CH 4 combustion [25]. They exhibit not only * Corresponding authors. E-mail addresses: ff@tf.uni-kiel.de (F. Faupel), oca@tf.uni-kiel.de (O.C. Aktas). 1 Both authors contributed equally. Contents lists available at ScienceDirect Materials Today Chemistry journal homepage: www.journals.elsevier.com/materials-today-chemistry/ https://doi.org/10.1016/j.mtchem.2020.100251 2468-5194/© 2020 Elsevier Ltd. All rights reserved. Materials Today Chemistry 16 (2020) 100251