Synthesis and characterization of Ce-substituted hydroxyapatite by sol–gel method Omer Kaygili a , Sergey V. Dorozhkin b, ⁎, Serhat Keser c a Department of Physics, Faculty of Science, Firat University, 23119 Elazig, Turkey b Kudrinskaja sq. 1-155, Moscow 123242, Russia c Department of Chemistry, Faculty of Science, Firat University, 23119 Elazig, Turkey abstract article info Article history: Received 25 November 2013 Received in revised form 20 April 2014 Accepted 6 May 2014 Available online 22 May 2014 Keywords: Hydroxyapatite Cerium substitution Sol–gel method Bioceramics Both undoped hydroxyapatite (HAp) and three Ce-substituted HAp samples with variable amounts (from 0.5 to 2 at.%) of Ce were synthesized by sol–gel method. The samples were studied by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy to determine the crystallite size, crystallinity degree, phases, functional groups, morphology and elemental composition. In all samples, the amount of HAp exceeded 92%, while the amount of admixture β-TCP was always below 8% and no changes were observed by addition of Ce. The crystallinity degree of the sam- ples was always within 84–89%, while the calculated dimensions of crystallites appeared to be within 26–35 nm. The microstructure and elemental composition of all the samples were found to be affected by the ad- dition of Ce. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Hydroxyapatite (HAp), with the chemical formula of Ca 10 (- PO 4 ) 6 (OH) 2 and Ca/P ratio 1.67, is one of the most popular bioceramic materials [1–3]. Since it resembles the major inorganic constituent of human bones and teeth [4], HAp implants possess the eminent properties such as non-toxicity, biochemical tolerance, bioactivity and biocompatibility. Therefore, HAp bioceramics have been widely used in medicine including dentistry [5] and orthopedic applications [1–3]. Under the physiological temperature and pH con- ditions, HAp is a sparingly soluble compound [6,7]. In addition, due to the above-mentioned prominent properties, various types of HAp-based composites, coatings and thin films have been extensive- ly developed and used for biomedical applications by many re- searchers [8–13]. Various techniques such as sol–gel synthesis, solid-state reactions, chemical precipitation, spray pyrolysis, combustion synthesis, mecha- nochemical route, microemulsion and microwave synthesis have been developed to synthesize HAp [14,15]. Among the available methods, the sol–gel technique is used to prepare nanostructured HAp of high pu- rity and crystallinity [16–18]. Furthermore, HAp and other calcium or- thophosphates might be easily doped with various cations and anions to improve their microstructure, biocompatibility and mechanical properties [19–25]. Cerium (Ce) can act similar to calcium in organisms, so it accumulates in bones in small amounts, therefore, Ce-containing compounds can stim- ulate metabolism in organisms [26,27]. The electronegativity of metallic Ce is 1.06, while the ionic radius of Ce 3+ is 0.107 nm. Both values are close to those of Ca (1.01 and 0.100 nm for Ca 2+ , respectively). Therefore, Ce 3+ may replace Ca 2+ in the lattice of HAp. In addition, trivalent Ce 3+ cations possess some antibacterial properties, which are beneficial for biomedical applications [28]. In the present work, we used a sol–gel method to synthesize both undoped HAp and Ce-doped HAp samples with variable amounts of Ce. The effect of Ce on the crystal structure, morphology and elemen- tal composition of the HAp samples was investigated by X-ray dif- fraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy techniques. Thus, we intended to investigate whether the addition of Ce, which was found to affect the solubility of HAp [28], could influence the crystallite dimensions and micro- structure, which might become a crucial starting point in further studies for biomedical applications of the Ce-containing calcium orthophosphates. 2. Materials and methods Samples of both undoped HAp and Ce-doped HAp were prepared by a sol–gel route. The amounts of Ce were 0 (undoped HAp), 0.5, 1.0 and 2.0 at.% and the samples were referred to as H1, H2, H3 and H4, Materials Science and Engineering C 42 (2014) 78–82 ⁎ Corresponding author. E-mail addresses: okaygili@firat.edu.tr (O. Kaygili), sedorozhkin@yandex.ru (S.V. Dorozhkin), skeser@firat.edu.tr (S. Keser). http://dx.doi.org/10.1016/j.msec.2014.05.024 0928-4931/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec