Solar Energy 254 (2023) 158–167 Available online 17 March 2023 0038-092X/© 2023 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. Effect of surface treatments on absorptance and morphology of molybdenum and silica absorbing thin flms Aline da Silva Oliveira a, * , Gustavo C´ esar Pamplona de Sousa a , Antˆ onio Karlos Araújo Valença a , Jos´ e Felix da Silva Neto b , Kelly Cristiane Gomes b a Department of Mechanical Engineering, Federal University of Paraíba, Jo˜ ao Pessoa, Brazil b Department of Renewable Energy Engineering, Federal University of Paraíba, Jo˜ ao Pessoa, Brazil A R T I C L E INFO Keywords: Solar collectors Absorber flms Surface treatment Sputtering Electropolishing ABSTRACT The thermal performance of solar collectors depends on the selective coverage over their absorbing plate, known as selective surfaces, which determines the heat gain and loss of the equipment. Different materials and layering arrangements can be used for the production of selective surfaces. This work proposes the production of selective surfaces of molybdenum (Mo) and silica (SiO 2 ) evaluating the infuence of the type of substrate treatment (chemical cleaning, passivation and electropolishing) on this performance. The results obtained in UV–Visible and Near Infrared Spectrophotometry showed that the surface absorptance is higher for Mo and Mo/SiO 2 flms on electropolished surfaces compared to treatments with acid and hexane. The highest absorptance reached was 98.10% for a Mo/SiO 2 flm (45.023 nm), on electropolished substrate. However, the highest solar selectivity was obtained from the Mo/SiO 2 flm on a passivated substrate. X-ray diffraction (XRD) exhibited peaks characteristic of the metallic Mo phase, and the presence of silica in the amorphous and crystalline phases in the form of quartz was also verifed. The results obtained in Optical Proflometry and Atomic Force Microscopy (AFM) indicate that the absorptance of the samples is infuenced by the roughness of the substrates. 1. Introduction Selective surfaces are essential for improving the performance of photothermal solar collectors. In the utilization of solar radiation, this equipment is used for heating water and represent a means of obtaining thermal energy at a reasonable cost, avoiding greater damage to the environment (Manea et al., 2010). While solar thermal conversion is cost-effective, there are limits to this application when working fuids are required to be fed at temperatures above 100 ◦ C (Gomes et al., 2002). In order to reduce heat loss through radiation and increase the thermal stability of the absorbing surfaces, several works (Barshilia et al., 2007; Gao et al., 2016, 2017; Gomes et al., 2002; Ibrahim et al., 2018; Ning et al. 2021; Rahman et al., 2016; Rebouta et al., 2015; Sel- vakumar and Barshilia, 2012; Wu et al., 2015; Zheng et al., 2015; Silva Neto et al., 2019) have been carried out using a variety of materials and confgurations for selective surfaces. The materials used to obtain selective surfaces are fundamental to the fnal properties of these surfaces. Among them, molybdenum is a transition metal with a high melting point, approximately 2610 ◦ C, and a thermal expansion coeffcient well below that most steels, providing dimensional stability at elevated temperatures, in addition to having good thermal conductivity and a specifc low heat, which ensures that the material is free from high thermal stresses when heated and cooled rapidly (Souza and Lima, 2016). Regarding the confguration of selective surfaces, the addition of the external dielectric layer (anti-refective) on the thin metallic coating reduces radiation losses in the infrared region and acts as a refection attenuator on the surface of this coating. Usually used for this purpose, silicon dioxide is classifed as a type of silicate, which are composed mainly of silicon and oxygen, the most abundant elements in the Earth’s crust. From an optical point of view, this material allows the trans- mission of visible light with a refractive index of 1.46. The Si-O inter- atomic bonds are covalent, making them relatively strong, resulting in a high melting temperature of 1710 ◦ C. In addition, it is a material with high thermal and chemical stability (Callister and Rethwisch, 2016; * Corresponding author. E-mail addresses: aline.oliveira@cear.ufpb.br (A. da Silva Oliveira), pamplonagustavo@hotmail.com (G. C´ esar Pamplona de Sousa), akavalenca@gmail.com (A. Karlos Araújo Valença), josefelix@cear.ufpb.br (J. Felix da Silva Neto), gomes@cear.ufpb.br (K. Cristiane Gomes). Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener https://doi.org/10.1016/j.solener.2023.03.001 Received 9 February 2022; Received in revised form 28 February 2023; Accepted 1 March 2023