Reduction dependent wetting properties of graphene oxide Francesco Perrozzi a , Salvatore Croce a , Emanuele Treossi b,c , Vincenzo Palermo b , Sandro Santucci a , Giulia Fioravanti a , Luca Ottaviano a,d, * a Dipartimento di Scienze Fisiche e Chimiche, Universita ` dell’Aquila, Via Vetoio, 67100 L’Aquila, Italy b CNR-ISOF, Via Gobetti 101, 40129 Bologna, Italy c Laboratorio MIST.E-R, Via Gobetti 101, 40129 Bologna, Italy d CNR-SPIN UOS L’Aquila, Via Vetoio, 67100 L’Aquila, Italy ARTICLE INFO Article history: Received 27 February 2014 Accepted 20 May 2014 Available online 28 May 2014 ABSTRACT This study reports contact angle measurements of standard, diol and aromatic solvents on graphene oxide thin films thermally reduced in ultra-high vacuum up to 900 °C. The films were chemically and morphologically characterized using respectively X-ray photoemis- sion spectroscopy and atomic force microscopy. The characterization shows that the wet- ting occurs in the chemically heterogeneous regime, namely the surface roughness (3 nm) does not influence the wetting properties of the samples. Zisman, Owens–Wendt and Neumann methods have been applied in order to calculate the surface free energy of the thin films showing that the Owens–Wendt method best fit the data trends. The surface free energy varies from 51 mN/m (pristine graphene oxide) to 39 mN/m (900 °C reduced graph- ene oxide). A correlation between the surface chemical composition, the surface free energy and its polar and dispersive components is reported, giving a rationale to the wet- ting properties of graphene oxide and reduced graphene oxide. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Graphene oxide (GO), the oxidized form of graphene, is a well- known two-dimensional material resulting from the chemical exfoliation of graphite [1]. At first proposed as a low cost pre- cursor of graphene with scalability and low cost production potential, it has progressively become important on its own thanks to its peculiar characteristics. Indeed, though far from the outstanding physical properties of graphene, GO has its interesting properties such as tuneable optoelectronic proper- ties thanks to reduction [2], remarkable mechanical strength [3], good water and organic solvents solubility [4] and amph- iphilicity [5] and catalytic properties [6,7] GO (and reduced GO (rGO)) is therefore attractive in electro-optical [8–10], filter- ing [11] and biological [12] applications as well as in the con- struction of gas sensing devices [13,14] and composite material [15]. In some specific applications (nano-biology, coatings, printable electronics) it is fundamental the knowl- edge of the GO and rGO interaction with their environment (typically the solvent where the GO is diluted), i.e. the knowl- edge of solvent compatibility/repellency with GO surfaces. The interfacial behaviour can be analysed studying the wet- ting properties of such systems. In particular the surface free energy (SFE) is the fundamental physical parameter that char- acterizes the wetting properties of solids surfaces. The SFE is usually determined by contact angles (CA) measurements. In http://dx.doi.org/10.1016/j.carbon.2014.05.052 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: luca.ottaviano@aquila.infn.it (L. Ottaviano). CARBON 77 (2014) 473 – 480 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon