Materials Chemistry and Physics xxx (2015) 1e12
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Graphene oxide powders with different oxidation degree, prepared by synthesis variations of the Hummers method Jesus Guerrero-Contreras, F. Caballero-Briones* Instituto Polit ecnico Nacional, Laboratorio de Materiales Fotovoltaicos, CICATA Altamira, Km 14.5 Carretera Tampico-Puerto Industrial Altamira, 89600 Altamira, Mexico
h i g h l i g h t s Powders of graphene oxide with different oxidation degree were prepared through variations of the Hummers method. Raman spectroscopy and XRD demonstrated similar crystallite domain size in the samples. Electrical resistance, exfoliation degree and optical absorption depend on the molecular structure. Relation between synthesis, molecular structure and properties of GO powders was established.
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Article history: Received 19 June 2014 Received in revised form 18 November 2014 Accepted 2 January 2015 Available online xxx
Graphene oxide (GO) powders with different oxidation degree estimated through the relative intensity of the infrared absorption bands related to oxygen containing groups were prepared through variations of the Hummers method. The GO powders were analyzed by Transmission Electron Microscopy, Energy dispersive spectroscopy, X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, Raman spectroscopy, X-ray Diffraction, UVeVIS spectroscopy and Electrical Resistance measurements. Several square micron GO sheets with low wrinkling were obtained. Oxygen to carbon ratio is around 0.2 in all the samples although a strong variance in the relative intensity of the oxygen related infrared bands is evident. Thus, the oxidation degree was estimated from the FTIR measurements using the quotient between the CeO related bands area to the total area under the spectra. FTIR shows presence of hydroxyl (eOH), epoxy (CeOeC), carboxyl (eCOOH) and carbonyl (C]O) moieties and evidence of intermolecular interactions between adjacent groups. These interactions influence the exfoliation degree, the absorbance of the GO suspensions, as well as the electrical resistance, while the crystalline domain sizes, estimated from XRD and Raman do not show a noticeable behavior related with the composition and molecular structure. The results indicate that the electrical resistance is influenced mainly by the surface chemistry of the GO powders and not only by the O/C ratio. The control of the surface chemistry of GO powders would allow their use as additives in organic bulk heterojunction solar cells with enhanced photoconversion efficiency. © 2015 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide Chemical synthesis Molecular structure Electrical characterization
1. Introduction Graphene is a two dimensional material made of sp2 hybridized carbon, densely packed in a honeycomb structure. As the building block of graphite, graphene was theoretically predicted in 1940 [1] and in 2004, Novoselov and Geim reported a method to prepare single graphene sheets [2]. Between other properties, graphene
* Corresponding author. E-mail address:
[email protected] (F. Caballero-Briones).
absorbs photons from the visible to the infrared range with strong intra-band transitions and ultrafast response [3]. This feature makes graphene attractive for photovoltaic applications. Graphene can be prepared by mechanical exfoliation or chemical vapor deposition, although they are not suitable for bulk scale manufacture of graphene sheets [4]. On the other hand, graphene oxide (GO) is a graphene-based material which exhibits fascinating chemical, optical and electrical properties due both to the graphene skeleton and the oxygen containing functional groups located on the basal plane or at the sheet edges [5]. Graphene oxide rises enormous interest by their
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Please cite this article in press as: J. Guerrero-Contreras, F. Caballero-Briones, Graphene oxide powders with different oxidation degree, prepared by synthesis variations of the Hummers method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.01.005
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properties brought by its abundant oxygen-containing groups that lead to its hydrophilicity and facile modification to produce other graphene-based materials [6]. It can form stable aqueous colloids that would facilitate the assembly of macroscopic structures by simple and cheap solution processes. GO is an attractive alternative to CVD graphene in many applications, as much as electrical conductivity and optical band gap in GO can be modulated through the control of the functional groups [7]. Moreover, it is possible to access to zero-band gap graphene via complete removal of oxygencontaining functional groups [8], as well by the substitution of these groups with metallic or semiconductor nanoparticles [9]. Recent studies point out the importance of the oxygen-to-carbon ratio and the molecular structure of GO, as they influence physical properties such as the microwave absorption capability [10]. Graphene oxide has been recently used in organic solar cells to increase its efficiency [11,12]. Soluble P3HT-grafted graphene for efficient bilayer heterojunction photovoltaic devices was analyzed by Yu et al. [13]. GO was prepared using the modified Hummers method and functionalized with P3HT through the GO terminal carboxylic groups. The resulting G-P3HT was easily dispersed in common organic solvents and a three fold increase in photocurrent density was observed compared with cells prepared only with P3HT [13]. In recent experiments [14] GO prepared in our group has been tested as an additive to PEDOT:PSS layer in different weight ratios (PEDOT:PSS:GO) in P3HT:PCBM bulk heterojunction solar cells, and a noticeable increase of PCE compared with a sample without GO was found. Therefore the control of oxidation ratio as well as the functional groups for the application of the GO-derived materials in organic solar cells appear as crucial [15]. The initial efforts to understand the structure of graphene oxide were done by Hoffman and Ruess [16,17]. Hoffman suggested a structure of repeated units of 1, 2-epoxides on the entire basalplanes on the graphene structure. Ruess proposed a made up of sp3 hybridized basal-planes as opposed to the sp2 hybridized system of Hoffman. Both models were deduced from chemical reactivity. From 13C and 1H magic-angle spinning nuclear magnetic resonance (MAS NMR), Lerf [5] postulated GO as consisting of aromatic regions with non-oxidized benzene rings and regions with
aliphatic six-membered rings with oxygen functionalities such as 1,2-epoxides and hydroxyl groups in the basal-plane and carbonyl, carboxyl and hydroxyl groups at the edges. A very recent observation of graphene oxide using high-resolution transmission electron microscopy (HRTEM) supports the Lerf model [18]. On the other hand, after an extensive study including elemental analysis, transmission electron microscopy, X-ray diffraction, 13C magicangle spinning NMR, diffuse reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and et al. [19] postulated a model that electron spin resonance, Szabo includes interlinked cyclohexane and aromatic regions within the GO structure that explains the wrinkles in TEM observations; the model includes the presence of tertiary OHs, 1,3-ethers in each side of the basal plane as well as phenolic groups, and also states that cyclic ketones and quinones can be formed on the hexagon ribbons where the C]C bonds are cleaved. Finally, in a recent model, Ajayan [20] suggested that GO does not exist as a static structure with a definite set of functional groups. The Ajayan model is held by the fact that GO prepared by chemical oxidation exhibits strong variance in its structure and degree of oxidation depending on the particular oxidants used, but also on the graphite source and reaction conditions [21]. GO is obtained from the oxidation of graphite powder in concentrated strong mineral acid with strong oxidizing agents. Over-oxygenated graphite salts are then subjected to hydrolysis followed by rising with water that yields an aqueous colloidal dispersion [17]. Solid GO can be recovered by drying the dispersion either in vacuum or atmospheric pressure at room temperature, freeze drying or heating in air to low temperatures (50e65 C), to prevent the thermal decomposition of GO [16]. Due to the low drying temperature, GO powders usually contain variable amounts of residual water. As stressed before, the O/C ratio in GO influences structural properties such as exfoliation level [22], number of layers [23], sheet size, number of defects, relative fraction of sp2 and sp3 domains [24] and concentration of functional groups, between other [25], that in correspondence would modulate the band structure and the possibility to produce photoluminescence [7], control
Table 1 Experimental conditions of GO preparation.
Please cite this article in press as: J. Guerrero-Contreras, F. Caballero-Briones, Graphene oxide powders with different oxidation degree, prepared by synthesis variations of the Hummers method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/ j.matchemphys.2015.01.005
J. Guerrero-Contreras, F. Caballero-Briones / Materials Chemistry and Physics xxx (2015) 1e12
electrical and mechanical properties [27] between other. Additionally, functional groups provide nucleation sites for further chemical modification such as decoration and functionalization [21]. In the present work graphene oxides with different oxidation degree were prepared using variations of the Hummers method by changing the concentration of the oxidizing agents NaNO3 and KMnO4 and the residence times at the different stages of the synthesis. The GO powders were studied by Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Raman spectroscopy, X-ray Diffraction (XRD), UVeVIS spectroscopy and Electrical Conductivity measurements (EC). A procedure to estimate the oxidation degree from EDS and FTIR is proposed and relationship of the studied properties with the oxidation degree is discussed.
2. Experimental details 2.1. Preparation of graphene oxide GO was prepared by variations of the Hummers method using synthetic graphite powder (
