Combining energy efficiency with self-cleaning properties in smart glass functionalized with multilayered semiconductors Corrado Garlisi a, b, *, 1 , Esra Trepci c, 1 , Reem Al Sakkaf a, b , Elie Azar c, ** , Giovanni Palmisano a, b a Department of Chemical Engineering, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates b Research and Innovation on CO 2 and H 2 (RICH) Center, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates c Department of Industrial and Systems Engineering, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates article info Article history: Received 4 March 2020 Received in revised form 10 June 2020 Accepted 11 June 2020 Available online 11 July 2020 Handling Editor: Cecilia Maria Villas B^ oas de Almeida Keywords: Multilayer structure Photocatalytic activity Energy efficiency Smart glass abstract We propose here a new approach based on a combined study of the self-cleaning and energy-efficiency properties of stratified WO 3 /CueTiO 2 coatings for glazing applications. The multifunctional performance was investigated by varying the number of layers deposited on the glass substrate (i.e., 1, 2, 4 layers), while energy efficiency was assessed using building energy modeling. Results indicate that the self- cleaning ability can be boosted by a multilayer configuration due to the inhibited charge recombina- tion and to a gradual rise in surface roughness and porosity with the increasing number of layers. In particular, the highest wettability and degradation of toluene under simulated solar light were achieved over the coating consisting of Cu-doped TiO 2 stacked with WO 3 in a 4-layer configuration. On the other hand, the analogous sample, but in the 2-layer configuration, was the most active sample in the oxidation of carbon monoxide. In terms of energy efficiency, the applied coatings led to important energy savings, exceeding 8% in total energy consumption and 18% in cooling loads. The best energy efficiency levels in the tested glazing configurations were achieved with single-layer WO 3 . However, its low visible trans- mittance reduced the amount of natural daylight entering the studied buildings, unintentionally increasing the electric lighting loads. The results highlight the importance of alternative configurations, such as Cu-doped TiO 2 in a dual-layer arrangement with WO 3 , which provide a better balance between energy efficiency and access to daylight, an important driver of occupant comfort and wellbeing. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction The development of smart glass has gained significant interest over the last decades, both in academic circles and the construction industry. A great deal of research along these lines has been devoted to increasing the energy efficiency of glazed units, which are often identified as the elements responsible for the majority of the heat exchange between outdoor and indoor environments. It has been estimated that ca. 50% of the total energy consumption within buildings is due to heat loss or gain through their windows (Aburas et al., 2019), making the efficient regulation of heat through the windows an absolute priority for maximizing the energy con- servation in the built environment. Smart windows rely on thin layers of material applied on their surface, which tune the optical response of the glass in the ultra- violet (UV), visible and near-infrared (NIR) regions, allowing to improve its thermal properties, while preserving the visual char- acteristics (i.e., high transparency in the visible region). However, current research in this area is not only developing advanced coatings with increasing energy-saving potential, but it is also focusing on the multifunctional aspect. Smart coatings can be designed so as to simultaneously enable for more than one prop- erty, making the glass multifunctional (Garlisi et al., 2020). In addition to energy efficiency properties, the self-cleaning ability can be imparted to the coating, mainly by the incorporation of TiO 2 - based materials, active under radiation, allowing to maintain a cleaner glass thorough the removal of organic contaminants and * Corresponding author. Department of Chemical Engineering, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, United Arab Emirates. ** Corresponding author. E-mail addresses: corrado.garlisi@ku.ac.ae (C. Garlisi), elie.azar@ku.ac.ae (E. Azar). 1 Both authors contributed equally. Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro https://doi.org/10.1016/j.jclepro.2020.122830 0959-6526/© 2020 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 272 (2020) 122830