Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Proteic sol-gel synthesis, structure and magnetic properties of Ni/NiO core- shell powders Rinaldo M. Silva a , Rafael A. Raimundo b,c , Willian V. Fernandes d , Sandro M. Torres d , Vinícius D. Silva d , João P.F. Grilo e , Marco A. Morales b , Daniel A. Macedo d, ⁎ a Department of Chemical Engineering, UFPB, 58051-900 João Pessoa, Brazil b Department of Theoretical and Experimental Physics, UFRN, 59078-970 Natal, Brazil c Mechanical Engineering Postgraduate Program, UFPB, 58051-900 João Pessoa, Brazil d Materials Science and Engineering Postgraduate Program, UFPB, 58051-900 João Pessoa, Brazil e Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal ARTICLE INFO Keywords: Chemical synthesis Sol-gel Nickel oxide Nickel Magnetic properties ABSTRACT Core-shell structured magnetic Ni/NiO powders were prepared by a proteic sol-gel route. Commercial gelatin and nickel nitrate were used as precursor materials. The synthesized material was calcined in air at 500 °C and further investigated by XRD, VSM and TEM. In order to investigate the effects of the structure on the magnetic properties, NiO powders were synthesized by three other methods for sake of comparison: citrate method, nitrate calcination and combustion method. XRD results revealed that the core-shell structured material is composed of 84.8 wt% NiO and 15.2 wt% Ni, while the samples from other methods are single phase. Hysteresis loop at room temperature showed a strong ferromagnetic behavior for samples prepared by proteic sol-gel and citrate methods. Powders from nitrate calcination and combustion showed weak ferromagnetic behavior most likely attributed to unpaired moments in their nanoparticles. The overall results showed that the proteic sol-gel method is a versatile chemical way to prepare Ni/NiO core-shell powders with high ferromagnetic signals. 1. Introduction Nickel oxide (NiO) is a versatile material that has found technolo- gical application in catalysis, gas sensor, solid oxide fuel cell anodes, supercapacitors and electrochromic windows [1–5]. Despite the fact that Bulk NiO is an antiferromagnetic (AF) material below a Néel temperature of 523 K [6], AF+ferromagnetic(FM)/asperomagnetic or superparamagnetic properties are exhibited when nanosized [7–9]. It has been postulated that this behavior is probably caused by un- compensated magnetic moments due to absence of cell periodicity at the nanoparticle surfaces and defects at nanoparticle cores [10,11]. On the other hand, bulk Ni exhibits FM properties below a Curie transition at 627 K. Therefore, Ni/NiO composites have been used in several technological applications combining intrinsic properties of each phase [12,13]. The effects of different synthesis routes on the particle size and magnetic properties of NiO and Ni/NiO composites have been widely reported in literature [9,12–15] showing that phase composition and particle morphology are key parameters to control magnetic perfor- mance. Accordingly, a structural study of NiO nanoparticles synthesized with commercial flavorless gelatin as an environmentally friendly precursor was reported elsewhere [16]. The gelatin is a natural polymer composed by a mixture of high molecular weight polypeptides and proteins usually obtained by hydrolysis of collagen. The present paper is the first report on the magnetic characteriza- tion of core-shell structured Ni/NiO powders synthesized by proteic sol- gel method using gelatin as chelating and polymerizing agent. Structure and magnetic properties of NiO samples prepared by citrate, nitrate calcination and combustion methods were also investigated. 2. Experimental Nickel oxide (NiO) powder samples were synthesized by four dif- ferent methods, namely: citrate, nitrate calcination, combustion and proteic sol-gel. The citrate synthesis started by dissolving nickel nitrate hexahydrate [Ni(NO 3 ) 2 ·6H 2 O, Sigma-Aldrich, 99%] in distilled water and further complexation with citric acid [C 6 H 8 O 7 ·H 2 O], using a acid – to – Ni molar ratio of 3.5. The resulting solution was stirred at 80–90 °C for 2 h to allow the formation of Ni-chelates. Afterwards, a thermal treatment was applied consisting of heating at 350 °C in air, for 1 h with https://doi.org/10.1016/j.ceramint.2017.12.248 Received 6 November 2017; Received in revised form 22 December 2017; Accepted 31 December 2017 ⁎ Corresponding author. E-mail address: damaced@gmail.com (D.A. Macedo). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Silva, R.M., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2017.12.248