Limestones coated with photocatalytic TiO 2 to enhance building surface with self-cleaning and depolluting abilities Angela Calia a, * , Mariateresa Lettieri a , Maurizio Masieri a , Sudipto Pal b , Antonio Licciulli b , Valentina Arima c a CNR IBAM (Institute of Archaeological Heritage), Prov.le Lecce-Monteroni, 73100 Lecce, Italy b Department of Engineering for Innovation, Salento University, Via Monteroni, 73100 Lecce, Italy c CNR NANOTEC, Via Monteroni, 73100 Lecce, Italy article info Article history: Received 30 December 2016 Received in revised form 26 July 2017 Accepted 26 July 2017 Available online 27 July 2017 Keywords: Photoactive TiO 2 Sol gel-hydrothermal synthesis Photocatalytic limestone surface Rhodamine B degradation NO x abatement Coating morphology abstract Natural stones with self-cleaning and depolluting abilities are appealing to preserve building façades in polluted urban sites and simultaneously to provide air-purification. Coating with photocatalytic Titanium dioxide is promising at this purpose; nonetheless, stone coating issues need better insights to support large-scale applications. In this paper, photocatalytic surfaces of two limestones having different roughnessess and porosities, are investigated by comparing coatings obtained from either water and alcohol based colloidal suspensions of TiO 2 nanoparticles, which were synthetized by sol gel and hy- drothermal process and sprayed with different loads on the stone surface. A commercial water-based TiO 2 sol was also used. The study aims to assess the role of the substrates, the nature of the titania dispersions and the TiO 2 loads, in determining characteristics and properties of the photocatalytic stone surfaces, in order to obtain suited coatings for real applications on buildings. Colorimetry detected negligible colour changes on both stone surfaces due to the coatings. A photodegradation test of Rhodamine B recorded a high self-cleaning efficiency on the coated surfaces, irrespective of the stones, the alcohol and water based suspensions, and their TiO 2 loads. Conversely, the efficiency in a NO x abatement test was dependent on the porosity and roughness of the stones. ESEM-EDS on the applied coatings and XRD on the TiO 2 nanopowders identified critical issues in the coating morphology and presence of by-products relating to the preparation of the sols, which may have implications in the durability performances. The overall results showed that all the obtained coatings were able to deliver photocatalytic surface of both limestones, which have a potential to be implemented as eco-efficient materials on buildings. Nonetheless, higher air purification ability issued for the limestone with higher porosity and roughness and the experimental TiO 2 water-based sol performed better than the alcoholic and commercial ones as regards the coating morphology and absence of by-products. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Heterogeneous photocatalysis related to nanostructured tita- nium dioxide, which is promoted by photo-excited electron-hole pairs in the TiO 2 semiconductor, is an eco-friendly process with effective applications in many fields for environmental protection and remediation against increasing pollution. It can be activated by the UV part of the solar light and it is able to degrade a variety of organic (Chong et al., 2015) and inorganic pollutants (Chen and Mao, 2007), as well as micro-organisms, thus promoting antibac- terial (Kikuchi et al., 1997) and antifungal effects (Vu ceti c et al., 2014). The technology of photocatalytic building materials, such as glass (Zhao et al., 2008), tiles (Radeka et al., 2014), heavy clay fired materials (Tobaldi et al., 2008), concrete pavements (Chen and Chu, 2011), finishing materials for urban buildings (Shen et al., 2015), hydraulic restoration mortars (Franzoni et al., 2014) and cement based architectural mortars (Guo et al., 2016), has been developed thanks to the use of titanium dioxide nanoparticles, and it has * Corresponding author. E-mail addresses: a.calia@ibam.cnr.it (A. Calia), mt.lettieri@ibam.cnr.it (M. Lettieri), m.masieri@ibam.cnr.it (M. Masieri), sudipto.pal@unisalento.it (S. Pal), antonio.licciulli@unisalento.it (A. Licciulli), valentina.arima@nanotec.cnr.it (V. Arima). Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro http://dx.doi.org/10.1016/j.jclepro.2017.07.193 0959-6526/© 2017 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 165 (2017) 1036e1047