Vol.:(0123456789) MRS COMMUNICATIONS · VOLUME XX · ISSUE xx · www.mrs.org/mrc 1 MRS Communications https://doi.org/10.1557/s43579-023-00440-4 Research Letter © The Author(s), 2023 Ciprofoxacin degradation with a defective TiO 2‑x nanomaterial under sunlight Nicola Bazzanella, Om Prakash Bajpai, Murilo Fendrich, Graziano Guella, Antonio Miotello, and Michele Orlandi , Department of Physics, University of Trento, Via Sommarive 14, 38123 Trento, Italy Address all correspondence to Michele Orlandi at michele.orlandi@unitn.it (Received 29 May 2023; accepted 9 August 2023) Abstract TiO 2 nanoparticles obtained by microwave-assisted hydrothermal synthesis were subjected to facile post-processing based on vacuum annealing. The resulting defective (TiO 2-x ) material showed extended light absorption in the visible range, enhancing sunlight compatibility. Structural and optical characterization point to the arising of an Urbach tail as the main reason. TiO 2-x was then employed as photocatalyst for the inactivation of ciprofoxacin, an emerging environmental threat and source of antimicrobial resistance, under sunlight. TiO 2-x performed signifcantly better than its parent compound and mechanistic analysis of the degradation pathway indicates that this material is a promising candidate for solar treatment of fuoroquinolones residues in water. Introduction Titanium dioxide is a benchmark material for heterogeneous photocatalysis, owing to a combination of good catalytic per- formance with remarkable chemical stability in a wide range of experimental conditions. [1] However, decades of research notwithstanding, actual applications to solar photocatalysis of this material are still limited because of poor, owing to a band structure only allowing absorption of the UV fraction of the spectrum. [2] A variety of methods have been developed over time to address this issue, including tweaking the electronic structure by doping, [3,4] sensitization by coupling with molecu- lar or plasmonic absorbers, [5,6] and the fabrication of compos- ites. [7,8] Another approach relies on inducing surface defects in the material which result in a smaller optical band gap and/or the rising of an Urbach tail. [9,10] To this scope, oxygen vacan- cies and Ti 3+ have been shown to be particularly efective for photocatalysis purpose. [11,12] A diferent approach to the issue of low compatibility between TiO 2 absorption and sunlight spectrum is that of working not on the material but on the illu- mination source, for example, by manipulating solar radiation using concentrating collectors. [13] In this case, the increas- ing availability of economically viable concentrator-based processes is an important progress towards applications. [14] The equipment to make an efective use of the UV fraction of sunlight remains signifcantly complex though, requiring for example especially designed mirrors based on anodized alu- minum. [15] Thus, improving the optical matching between TiO 2 and sunlight would be greatly benefcial also in this context. In this work, we investigated a facile modifcation of a TiO 2 nanomaterial, resulting in extended light absorption in the visible range. A microwave-assisted hydrothermal synthesis was employed to obtain TiO 2 nanopowders, which were then annealed in vacuum. The resulting material (TiO 2-x ) shows a color change to light-brown, a decreased, albeit modestly opti- cal band gap, but especially the arising of an absorption tail in the visible range. Optical and structural properties were inves- tigated by UV–Vis and micro-Raman spectroscopy, electron microscopy both in scanning (SEM) and transmission (TEM) mode, and selected area electron difraction (SAED). To ver- ify the suitability of this material for solar photocatalysis, the degradation of Ciprofoxacin (CIP) under direct sunlight was studied and a reaction mechanism investigated by determin- ing intermediates with High-Performance Liquid Chroma- tography (HPLC) methods. CIP, a fuoroquinolone antibiotic used in both medicine and veterinary medicine, was selected as the target pollutant for several reasons: (a) it is one of the most used fuoroquinolones worldwide, employed in massive quantities; (b) current wastewater treatment processes are only marginally efective at removing CIP, and consequently, it has been detected in surface waters worldwide; (c) as such, it can heavily contribute to the rise of antimicrobial resistance (AMR), a top threat to public health. [16] In this framework, solar photocatalysis is a promising approach for a potentially cost-efective, sunlight-driven pollutant degradation process with high efciency. Also, it is worth considering that while the ultimate goal of solar photocatalysis treatments is generally considered as the total removal of pollutants via mineraliza- tion, AMR reduction in principle only requires the inactivation of antibiotic residues in water. This is, at least on paper, a less demanding goal. [17]