Use of Optical Contrast To Estimate the Degree of Reduction of Graphene Oxide Francesco Perrozzi, † Stefano Prezioso,* ,† Maurizio Donarelli, † Federico Bisti, † Patrizia De Marco, †,‡ Sandro Santucci, † Michele Nardone, † Emanuele Treossi, ¶,§ Vincenzo Palermo, ¶ and Luca Ottaviano † † Dipartimento di Fisica, Universita ̀ dell’Aquila, Via Vetoio 10, 67100 L’Aquila, Italy ‡ Chimie des Interactions Plasma Surface (ChIPS), UMONS Research Institute for Materials Science and Engineering, Universite ́ de Mons, Avenue N. Copernic 1, 7000 Mons, Belgium ¶ CNR-ISOF, via Gobetti 101, 40129 Bologna, Italy § Laboratory MIST.E-R, via Gobetti 101, 40129 Bologna, Italy * S Supporting Information ABSTRACT: We report an optical contrast study of graphene oxide on 72 nm Al 2 O 3 /Si(100) and 300 nm SiO 2 /Si(100) as a function of its reduction degree. The reduction has been performed by means of ultrahigh vacuum thermal annealing from 25 °C (pristine graphene oxide) to 670 °C. In parallel to the optical contrast investigation, performed with optical microscopy, the graphene oxide films have been characterized with core level X-ray photoemission spectroscopy and micro- Raman spectroscopy. The optical contrast of graphene oxide (normalized to the one measured for pure graphene) on both substrates ranges from ∼0.4 to 1.0 for pristine and 670 °C annealed graphene oxide, respectively. Optical microscopy and X-ray photoemission spectroscopy data have been cross- correlated, leading to calibration graphs that demonstrate that just by simply measuring the optical contrast of graphene oxide one can determine with very good approximation the fraction of sp 2 hybridized carbon. ■ INTRODUCTION Graphene oxide (GO), in a naive manner generally known as “the oxidized form of graphene”, came into the spotlight of the exploding graphene related research community as a possible low cost, large scale production, precursor of single and few layers graphene. 1 Although GO does not retain the outstanding qualities of graphene in terms of some physical properties like electron conductivity 2,3 or mechanical resistance, 4 which are of fundamental importance for some potential groundbreaking applications of graphene, it has its own specific tremendous potential applications in optics, 3,5,6 gas sensing, 7,8 composite materials and gas barriers, 9 and nanobiotecnology. 10 Various theoretical models can be found in the literature on GO. 11,12 All of them essentially suffer from oversimplification, as, once viewed in reality, 2,13,14 GO exhibits a fascinating “built-in” complexity. Essentially, GO is a two-dimensional inhomoge- neous system at the nanometer scale level. It typically shows at that length scale a phase separated landscape with holes (typically covering about 5% of the GO surface), patches of amorphous carbon (in reduced GO), parts with defective graphene functionalized with epoxy oxygen, carboxyl (OH), carboxylic (OC-OH), and carbonyl (OC) groups, and areas of pure graphene surviving from the chemical exfoliation of pristine nonoxidized graphite. The length scale of such features is a few nanometers (in the 1-5 nm range). The size and surface concentration of holes, pure graphene areas, and amorphous carbon patches, and also the type and concentration of functional groups, are ultimately related to the GO preparation methods and postexfoliation processes (like ultrahigh vacuum (UHV) thermal reduction or chemical reduction). Thus, at variance from the disruptively appealing physical properties of graphene, which are well-defined and substantially predictable from first principles, the physical and chemical properties of GO (thermal and electron conductivity, optical absorption coefficient, Young modulus, reduction degree, hydrophobicity, etc.) are essentially not well-defined even if, on the other hand, nicely tunable. A survey of the characterization studies of GO bearing the ambition of completeness is essentially hopeless, given the explosive nature of research on this material. An inherently limited review is thus given in the following. GO has been studied and characterized by an impressively great deal of experimental techniques and approaches. 15 Its functionalization is typically studied with X- ray photoemission spectroscopy (XPS) 1,16 or infrared spec- troscopy 17 and Raman spectroscopy (RS). 18 In particular RS, a pivotal characterization technique of graphene, has also its great Received: July 13, 2012 Revised: October 8, 2012 Published: December 3, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 620 dx.doi.org/10.1021/jp3069738 | J. Phys. Chem. C 2013, 117, 620-625