Citation: Kalyk, F.; Žalga, A.; Vasiliauskas, A.; Tamuleviˇ cius, T.; Tamuleviˇ cius, S.; Abakeviˇ cien ˙ e, B. Synthesis and Electron-Beam Evaporation of Gadolinium-Doped Ceria Thin Films. Coatings 2022, 12, 747. https://doi.org/10.3390/ coatings12060747 Academic Editors: Sheng-Rui Jian and Phuoc Huu Le Received: 8 April 2022 Accepted: 27 May 2022 Published: 29 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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 Synthesis and Electron-Beam Evaporation of Gadolinium-Doped Ceria Thin Films Fariza Kalyk 1,2, * , Art ¯ uras Žalga 3 , Andrius Vasiliauskas 2 , Tomas Tamuleviˇ cius 1,2 , Sigitas Tamuleviˇ cius 1,2 and Brigita Abakeviˇ cien˙ e 1,2 1 Department of Physics, Kaunas University of Technology, Studentu˛ St. 50, 51368 Kaunas, Lithuania; tomas.tamulevicius@ktu.lt (T.T.); sigitas.tamulevicius@ktu.lt (S.T.); brigita.abakeviciene@ktu.lt (B.A.) 2 Institute of Materials Science, Kaunas University of Technology, K. Baršausko St. 59, 51423 Kaunas, Lithuania; andrius.vasiliauskas@ktu.lt 3 Department of Applied Chemistry, Faculty of Chemistry and Geosciences, Vilnius University, Naugarduko St. 24, 03225 Vilnius, Lithuania; arturas.zalga@chf.vu.lt * Correspondence: fariza.kalyk@ktu.lt; Tel.: +37-062903862 Abstract: Gadolinium-doped ceria (GDC) nanopowders, prepared using the co-precipitation synthe- sis method, were applied as a starting material to form ceria-based thin films using the electron-beam technique. The scanning electron microscopy (SEM )analysis of the pressed ceramic pellets’ cross- sectional views showed a dense structure with no visible defects, pores, or cracks. The AC impedance spectroscopy showed an increase in the total ionic conductivity of the ceramic pellets with an in- crease in the concentration of Gd 2 O 3 in GDC. The highest total ionic conductivity was obtained for Gd 0.1 Ce 0.9 O 2-δ (σ total is 11 × 10 −3 S·cm −1 at 600 ◦ C), with activation energies of 0.85 and 0.67 eV in both the low- and high-temperature ranges, respectively. The results of the X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectrometer (ICP-OES) mea- surements revealed that the stoichiometry for the evaporated thin films differs, on average, by ~28% compared to the target material. The heat-treatment of the GDC thin films at 600 ◦ C, 700 ◦ C, 800 ◦ C, and 900 ◦ C for 1 h in the air had a minor effect on the surface roughness and the morphology. The results of Raman spectroscopy confirmed the improvement of the crystallinity for the corresponding thin films. The optimum heat-treating temperature for thin films does not exceed 800 ◦ C. Keywords: gadolinium-doped ceria; GDC; co-precipitation synthesis; electron-beam evaporation; thin films; SOFC; impedance spectroscopy 1. Introduction Miniaturized solid-oxide fuel cells (μ-SOFCs), constructed using thin-film technolo- gies, can achieve high specific energy and energy density and may, one day, partially replace Li batteries in portable devices [1–5]. However, the initial materials used in the fabrication of the μ-SOFC process should fully satisfy their requirements. Recently, the thickness of the μ-SOFC three-layered structure (anode-electrolyte-cathode) has been re- duced to a one-micron size. Thus, the thickness of the electrolyte thin film in μ-SOFC becomes thinner, e.g., ~600 nm, compared to conventional SOFC (~1 μm) [6–8]. This re- duced thickness can minimize the ionic transport path and significantly reduce the ohmic resistance [9]. The development of thin-film ceramic electrolytes over the past several decades has led to reduced operating temperatures for SOFCs [10]. Conventional materials, such as ceria or zirconia-based ceramics, are still widely used as electrolytes [2,11]. Due to their superior properties, such as high ionic conductivity and low activation energy [12], gadolinium-doped ceria (GDC) ceramics are widely applied in the production of μ-SOFC as an electrolyte [4], interlayer [9], or in the composition of an anode [13]. GDC is one of the most promising electrolytes for μ-SOFC, with only one condition: that the operating Coatings 2022, 12, 747. https://doi.org/10.3390/coatings12060747 https://www.mdpi.com/journal/coatings