Tailoring the visible light photoactivity of un- doped defective TiO 2 anatase nanoparticles through a simple two-step solvothermal process David G Calatayud 1,5 , Raquel M Flores 1,2 , Ana Castellanos-Aliaga 1,3 , Marco Peiteado 1 , Francisco J Palomares 4 , Amador C Caballero 1 and Teresa Jardiel 1 1 Department of Electroceramics, Instituto de Cerámica y Vidrio—CSIC, Kelsen 5, Campus de Cantoblanco, E-28049, Madrid, Spain 2 Superior School of Experimental Science and Technology, Universidad Rey Juan Carlos, Tulipán, s/n, E-28933 Móstoles, Madrid, Spain 3 Department of Inorganic Chemistry, Universidad Autónoma de Madrid, Francisco Tomás y Valiente 7, Campus de Cantoblanco, E-28049 Madrid, Spain 4 Department of Nanostructures and Surfaces, Instituto de Ciencia de Materiales de Madrid—CSIC, Sor Juana Iné s de la Cruz 3, Campus de Cantoblanco, E-28049 Madrid, Spain E-mail: dgcalatayud@icv.csic.es Received 20 May 2019, revised 6 September 2019 Accepted for publication 1 October 2019 Published 23 October 2019 Abstract Anatase TiO 2 has become a material of great interest for photocatalytic production of hydrogen, environmental purification and solar energy conversion. Among the key parameters boosting the photocatalytic efficiency of the anatase nanoparticles, an increased light absorption to expand its optical response to the visible region, together with an improved charge separation of the photo- generated electrons and holes, can be enumerated. In this work, yellow-coloured, single-phase anatase nanoparticles have been obtained using a simple two-step solvothermal routine which requires no external addition of dopants, nor the use of a harassing/aggressive synthesis atmosphere. The obtained powders display a lowered bandgap (<3.0 eV) and significantly reduce the recombination processes, eventually leading to an improved photocatalytic performance under visible light, as exemplified by an enhanced degradation of phenol. This exceptional response is linked to the presence of intrinsic defects in the yellowish particles and, hence, the specific conditions of the proposed methodology become crucial to produce a propitious TiO 2 -defective nanomaterial capable of photo-degrade the phenol molecule, in contrast with the lack of photocatalytic activity currently exhibited by commercial photocatalysts under visible light. Keywords: anatase, solvothermal synthesis, photocatalysis, nanoparticles, bandgap (Some figures may appear in colour only in the online journal) Introduction Semiconductor nanomaterials find a wide range of applica- tions in the area of photocatalysis where they can be used for environmental purification, hydrogen generation or solar energy conversion. So far, titanium oxide is the most inves- tigated among the semiconductor photocatalysts and this can be mainly attributed to its non-toxicity, abundance and inexpensiveness, but also to its improved thermal/chemical stability and high redox potential [1–7]. Accordingly a lot of Nanotechnology Nanotechnology 31 (2020) 045603 (12pp) https://doi.org/10.1088/1361-6528/ab49af 5 Author to whom any correspondence should be addressed. 0957-4484/20/045603+12$33.00 © 2019 IOP Publishing Ltd Printed in the UK 1