Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Absorption boost of TiO 2 nanotubes by doping with N and sensitization with CdS quantum dots Andjelika Bjelajac a, ⁎ , Veljko Djokić b , Rada Petrović b , Nenad Bundaleski c , Gabriel Socol d , Ion N. Mihailescu d , Zlatko Rakočević c , Djordje Janaćković b a University of Belgrade, Innovation center of Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia b University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia c University of Belgrade, Vinča Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia d National Institute for Lasers, Plasma, and Radiation Physics, Lasers Department, "Laser-Surface-Plasma Interactions" Laboratory, PO Box MG-54, RO- 77125 Magurele, Ilfov, Romania ARTICLE INFO Keywords: A. MAPLE B. Nanocomposites C. Optical properties D. TiO 2 ABSTRACT A process of obtaining N-doped TiO 2 nanotubes sensitized by CdS nanoparticles is presented, including detailed characterizations performed along the synthesis. Transparent TiO 2 films consisting of nanotubes, 2.5 μm long and of ~60 nm inner diameter, were obtained after anodization of a titanium film deposited onto FTO glass substrate. N-doping was achieved by annealing of TiO 2 film in ammonia. X-ray Photoelectron Spectroscopy measurements showed that nitrogen was substitutionally incorporated in the TiO 2 matrix, with the N:Ti concentration ratio of 1:100. The doping changed the optical properties of the material in such a way that the absorption edge was shifted from 380 nm to 507 nm, as observed from diffuse reflectance spectra. The influence of the microwave (MW) irradiation on the synthesized CdS quantum dots and their optical properties was investigated. It was shown that the diameter of CdS nanoparticles was increased due to releasing of S 2- ions from dimethyl sulfoxide (DMSO) as a consequence of the MW treatment. The (N)TiO 2 films were then used as substrates for matrix assisted pulsed laser deposition of the CdS quantum dots with DMSO as a matrix. The laser parameters for the deposition were optimized in order to preserve the nanotubular structure open, the latter being an important feature of this type of photoanode. The structure obtained under optimized conditions has an additional absorption edge shift, reaching 603 nm. 1. Introduction The energy crisis is one of the major problems that today's civilization is facing. There are many studies showing that the problem can be manageable by using solar energy. The graphical presentation given in [1] shows that the newest, third generation of solar cells has the best cost/efficiency ratio. Among them, quantum dots sensitized solar cells (QDSSCs) are still challengeable for reaching their predicted high efficiency [2]. The photovoltaic (PV) effect in QDSSC originates from the TiO 2 based photoanode sensitized with quantum dots (QDs). The main advantage of QDs versus the alternative dye sensitizers is the multiple exciton generation (MEG), a process of converting the high energy photons into more than one electron [2]. Moreover, QDs are inorganic and robust, thus more stable as compared to the organic dyes. As reported elsewhere, CdS QDs are appropriate for sensitization of TiO 2 due to the energy levels’ position: the conductive band edge of bulk CdS is 0.5 eV above the conductive band edge of TiO 2 [3]. In case of QDs the conductive band edge is even higher than of CdS in bulk. Thus, there is a significant driving force for electron transfer from CdS to conductive band of TiO 2 , preventing the recombination of electrons with photogenerated holes within CdS [4]. There are two main scientific approaches in improving the perfor- mances of QDSSCs by TiO 2 processing. One of them focuses on providing high specific surface area, enabling more contacts between QDs and TiO 2 and decreasing the recombination of the carriers. It was shown that the electron transfer in TiO 2 nanotubes irradiated by UV light is 30 times faster than in the nanoparticles [5]. What is more, the carrier recombination and also the electron energy losses are reduced in nanotubes where there are no grain boundaries as between nanoparticles [6]. The anodization technique is widely used for obtaining highly ordered and perpendicularly oriented TiO 2 nanotubes onto Ti substrates [7]. To ensure the front side illumination of the solar cells with anodized TiO 2 nanotubes, Ti can be sputtered onto FTO glass and then used to fabricate transparent TiO 2 electrodes [8]. The second http://dx.doi.org/10.1016/j.ceramint.2017.08.029 Received 13 July 2017; Received in revised form 2 August 2017; Accepted 3 August 2017 ⁎ Corresponding author. E-mail address: abjelajac@tmf.bg.ac.rs (A. Bjelajac). Ceramics International 43 (2017) 15040–15046 Available online 04 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved. MARK