New water-based sol–gel synthesis routes for LaNi 0.6 Fe 0.4 O 3-δ thin films Martin Perz a, ⁎, Edith Bucher a , Werner Sitte a , Thomas Griesser b a Chair of Physical Chemistry, Montanuniversitaet Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria b Chair of Chemistry of Polymeric Materials, Montanuniversitaet Leoben, Otto Gloeckel-Straße 2, 8700 Leoben, Austria abstract article info Article history: Received 13 June 2014 Received in revised form 20 October 2014 Accepted 23 October 2014 Available online 11 November 2014 Keywords: Lanthanum iron nickelate Perovskite Solid oxide fuel cell cathode Thin films Spin coating Two synthesis routes for approximately 100 nm thick layers of LaNi 0.6 Fe 0.4 O 3-δ (LNF64) are presented. Aqueous solutions of acetate or nitrate-precursors were deposited on Ce 0.9 Gd 0.1 O 2-δ substrates by spin coating. After thermal annealing at 700 °C for 3 h, the thin films were characterized by X-ray diffraction, scanning electron mi- croscopy, and X-ray photoelectron spectroscopy. The DC electrical conductivity of the LNF64 thin films was determined by four-point van der Pauw measurements as a function of temperature (20 ≤ T/°C ≤ 400) at 1 × 10 -3 ≤ pO 2 /bar ≤ 1. At the surface and cross section a nanocrystalline microstructure with grain sizes of approximately 50–100 nm was observed. The DC electrical conductivity of the thin films increases with increasing temperature and increasing pO 2 . A maximum electrical conductivity of 320 S/cm was found at 400 °C and pO 2 = 1 bar which agrees with the literature data for LNF64 bulk samples. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Strontium or barium-substituted mixed conducting transition metal perovskites (La,Sr,Ba)(Co,Fe)O 3-δ have attracted considerable research interest during the last three decades due to their high electronic conductivity, high oxygen nonstoichiometry, and fast oxygen exchange kinetics, all of which make them attractive candidates for many applica- tions such as solid oxide fuel cell cathodes [1], oxygen permeable mem- branes or membrane reactors [2], and catalysts [3]. In recent years, an initiative to develop strontium and barium-free materials with similar properties has started because research results showed that the long- term degradation of materials from the series (La,Sr,Ba)(Co,Fe)O 3-δ is frequently correlated to Sr and Ba segregation to the surface, as well as the formation of undesirable reaction products like SrCrO 4 or (Ba,Sr)CO 3 with gas phase impurities which leads to a deactivation of the surface oxygen exchange properties [4,5]. In this respect LaNi 0.6 Fe 0.4 O 3-δ (LNF64) has been suggested as a promising candidate, with high electronic conductivity and a thermal expansion coefficient which matches that of the commonly applied solid electrolyte yttria- stabilized zirconia [6]. Nishi et al. reported that the surface oxygen exchange rate and the tracer diffusion coefficient of oxygen of LNF64 are comparable to those of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ [7]. Whether LNF64 also shows an improved long-term stability against Cr-poisoning is cur- rently under investigation [8]. While the synthesis of powders, as well as dense and porous macroscopic LNF64 samples, has been reported in various studies (for example [6,8,9]), few reports on the preparation of LNF64 thin films are available. Recently, Lacey et al. described the preparation of LNF64 thin films by utilizing physical vapour deposition (PVD) [10]. Compared to PVD and other methods such as chemical va- pour deposition (CVD), spin coating is more cost-effective and can be performed with relatively simple equipment [11]. Recently, the synthe- sis of La 0.6 Sr 0.4 CoO 3-δ nanolayers by a glacial acetic acid-based sol–gel spin coating method has been reported [12]. Dieterle et al. fabricated La 0.5 Sr 0.5 CoO 3-δ thin films using propionic and nonanoic acid as solvents [13]; however, these media have the disadvantage of being corrosive (which can lead to damage of the metal parts in the spin coater) and malodorous. In the present study, two water-based sol–gel processes for the preparation of LNF64 thin films by a spin coating method are introduced and the synthesised specimens are characterised with refer- ence to phase purity, microstructure, elemental composition, and DC electrical conductivity. Potential applications for these thin films in- clude electrodes for micro-solid oxide fuel cells [14], coatings for SOFC interconnectors [10], active layers for gas sensors or catalysts [15,16], and model electrodes for basic scientific investigations [12]. 2. Experiment 2.1. Preparation Sol–gel synthesis routes for LaNi 0.6 Fe 0.4 O 3-δ thin films with two dif- ferent precursor solutions were investigated. In the first approach, 38.00 mg of polyvinyl alcohol ((C 2 H 4 O) n , Fluka, b 10 mg/kg Cl - ) was dissolved in 3.5 ml of distilled H 2 O using an ultrasonic bath. Acetates of lanthanum (343.06 mg La(CH 3 COO) 3 .1.5H 2 O, Sigma-Aldrich, 99.9%), nickel (149.30 mg Ni(CH 3 COO) 2 .4H 2 O, Sigma-Aldrich, 99.99%), and iron (69.57 mg Fe(CH 3 COO) 2 , Sigma-Aldrich, 99.99%) as well as 0.2 ml of formamide (CH 3 NO, Sigma-Aldrich, puriss. p.a.) were added to the Solid State Ionics 273 (2015) 30–34 ⁎ Corresponding author. Tel.: +43 3842 402 4821; fax: +43 3842 402 4802. E-mail address: martin.perz@unileoben.ac.at (M. Perz). http://dx.doi.org/10.1016/j.ssi.2014.10.027 0167-2738/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi