SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2006; 38: 789–792 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.2259 ZnO thin ﬁlms on aluminized steel by spray pyrolysis R. L ´ opez Ib ´ a˜ nez, R. Romero, F. Mart ´ ın, J. R. Ramos-Barrado and D. Leinen ∗ Laboratorio de Materiales y Superﬁcie, (Unidad Asociada al CSIC), Dpto. F´ ısica Aplicada I & Dpto. Ingenier´ ıa Qu´ ımica, Universidad de M ´ alaga, 29071 M´ alaga, Spain Received 4 July 2005; Revised 25 November 2005; Accepted 29 November 2005 Spray pyrolysis is a low-cost technique and results in minimal waste production when using adequate precursors. The process allows the coating of large surfaces and is appropriate for adaptation into an industrial production line. The aim of this work has been to obtain, by spray pyrolysis, thin and compact ZnO ﬁlms on 30 × 40 cm galvanized steel sheets heated by high frequency magnetic induction and using zinc acetate precursor in aqueous spray solution. The work includes a study of optimized deposition conditions (substrate temperature, precursor solution concentration, ﬂow rate and spray time) controlled ex situ by SEM, XPS, XRD and UV-Vis-NIR and FTIR diffuse reﬂectance spectroscopy, in order to obtain an effective barrier coating with low thermal emittance when the steel is used as a base material for solar thermal devices. Our results show improved anti-corrosion performance when the substrate coverage is guaranteed, substrate temperature and deposition time being the main factors to accomplish the optical requirements of the ZnO-coated galvanized steel. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: spray pyrolysis; ZnO thin ﬁlm; aluminized steel sheet; physical barrier INTRODUCTION Galvanized steel sheets are being tested as base materials for absorber surfaces in unglazed solar collectors to be integrated in building fa¸ cades (EU project SOLABS; www.solabs.net). Because the collector will be directly exposed to the environment, it is necessary to improve its anti-corrosive behaviour. Usually a thick polymer coating or an inorganic layer could be used as an anti-corrosion barrier, 1 but current chromate coatings are extremely toxic and the speciﬁc optical requirements of the collector require using a very thin ﬁlm as an anti-corrosion barrier so that the thermal emittance of the whole absorber-barrier-substrate system does not increase. Without prejudice to the low thermal emittance, an improvement of the solar absorptance would be welcomed. Some metallic oxides comply with these requisites. 2 On the other hand, hot-dip aluminized steel has shown adequate anti-corrosive and optical properties to be used as a base material for unglazed solar thermal devices. 3 Nevertheless, it is necessary to improve its corrosion protection with a top barrier layer to delay early degradation. Spray pyrolysis is a low-cost technique that allows coating large surfaces. In this technique, the substrate to coat is kept at constant temperature while the precursor solution is atomized over it. To avoid the oxidation of the galvanized steel, low substrate temperatures must be used. On the other hand, the oxide thin ﬁlm obtained must show good adhesion to the galvanized steel and compatibility L Correspondence to: D. Leinen, F´ ısica Aplicada I, Facultad de Ciencias, Universidad de M´ alaga, E-29071 M´ alaga, Spain. E-mail: dietmar@uma.es Contract/grant sponsor: EU project SOLABS; Contract/grant number: ENK6-CT2002-00679. with the compounds forming the optical selective layers. Attending to these reasons, we have selected ZnO as a physical barrier. ZnO is an n-type semiconductor, with an optical bang-gap of 3.3 eV. 4,5 This metallic oxide has been used as an anti-reﬂection layer, and as a transparent conductor. 6,7 Prior to this work, we have obtained ZnO thin ﬁlms by spray pyrolysis on other substrates such as fused silica, silicon or glass, 8,9 as well as compact, transparent and well-adhered ZnO ﬁlms on aluminized steel substrate. 2 It is accepted that when the ZnO is directly exposed to moisture, it reacts with the water, resulting in the formation of zinc hydroxide. Over time, and under the inﬂuence of cyclic weathering, the ﬁnal corrosion product is zinc carbonate. Zinc carbonate forms a thin, tenacious, compact, stable and water-insoluble ﬁlm. This protective ﬁlm serves as a barrier between the moisture and the galvanized steel underneath. Electrochemical techniques have been used to determine the anti-corrosive properties of the ZnO thin ﬁlms deposited on the aluminized steel. 10 In H 2 SO 4 electrolyte solution, the corrosion rate can be measured from potentiodynamic polarization curves. This parameter is used in this work to estimate the anti-corrosion behaviour of zinc oxide layers on aluminized steel sheets in comparison with the substrate itself. EXPERIMENTAL For these purposes, a new spray station was speciﬁcally designed to cover larger surfaces, using previous experience in the spray pyrolysis technique. Four high frequency induction coils were employed to heat the steel, arranging them suitably to ensure a uniform thermal distribution; the substrate temperature was on trial at 200 ° C and 250 ° C. Copyright  2006 John Wiley & Sons, Ltd.