Article pubs.acs.org/cm
Stepwise Reduction of Immobilized Monolayer Graphene Oxides Søren Petersen,† Yudong He,‡ Jiang Lang,‡ Filippo Pizzocchero,§ Nicolas Bovet,† Peter Bøggild,§ Wenping Hu,‡ and Bo W. Laursen*,† †
Nano-Science Center & Department of Chemistry, University of Copenhagen, Copenhagen, Denmark Institute of Chemistry, Chinese Academy of Science, Beijing, China § DTU Nanotech, Technical University of Denmark, Lyngby, Denmark ‡
S Supporting Information *
ABSTRACT: Chemically converted graphene is highly relevant for transparent conducting film applications such as display and photovoltaic uses. So far, the major obstacle for realizing the potential has been to fully reduce/deoxygenate the graphene oxide (GO), which is challenging in part due to the pronounced aggregation that accompanies deoxygenation of GO in solution. Surface immobilization of monolayered graphene oxide (mGO) in Langmuir−Blodgett (LB) films was investigated as a method to circumvent this problem. Two types of LB films with different density of mGO flakes were prepared, i.e., diluted and coherent, and efficiently deoxygenated in a three-step reduction procedure involving subsequent treatment with hydrazine in dimethylformamide (DMF), sulfuric acid, and high temperature annealing. The stepwise reduction process was evaluated with optical microscopy, Raman microscopy, and X-ray photoelectron spectroscopy (XPS) along with electrical characterization. XPS measurements confirmed a full conversion into virtually oxygen-free chemically converted graphene. The electrical characterization revealed large variations in the conductivity for single sheets in the diluted LB films, with an average conductivity of 100 S/cm. A similar conductivity was found for macroscopic devices made from the coherent LB films with overlapping mGO sheets. The large variation in single sheets conductance is assigned to overoxidation of the GO leading to formation of holes, which cannot be recovered in the chemical reduction procedure. The study shows that the applied three-step reduction procedure is chemically complete and that the conductivity of this chemically converted graphene is limited by structural defects/holes rather than remaining oxygen functionalities. KEYWORDS: graphene, reduced graphene oxide, chemically converted graphene oxide, Langmuir−Blodgett
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hydrazine being the most common,21 and/or (2) a deoxygenation or decomposition step by high temperature annealing. The use of strong acid or base to deoxygenate GO has also been demonstrated.15,16,22 Meanwhile, these one- or two-step conversion schemes have proven insufficient for complete removal of all the oxygen-functionalities.23 In 2009, Gao et al. showed that a three-step conversion scheme was able to convert bulk GO back into an amorphous graphite-like material with an oxygen content of less than 0.5 wt %.24 This three-step reduction scheme used sodium borohydride as reducing agent and introduced an intermediate deoxygenation step involving treatment with concentrated sulfuric acid before performing a high-temperature annealing. Unfortunately, the bulk conversion results in irreversible aggregation of the graphene sheets. A similar but slightly milder scheme involving hydrazine as reducing agent, Fe/HCl as an intermediate step, and low
INTRODUCTION Since first isolated in 2004,1 graphene, an atomically thin, transparent, and highly conductive carbon allotrope,2 has been predicted to pave the way for new carbon electronics technologies.3 One of the major driving forces for this development is the limited availability and high cost of the widely used indium tin oxide (ITO) for transparent electrodes and the need for development of flexible electrodes for future electronics. With graphene’s unique combination of electrical, optical, and mechanical properties,4−7 it is an obvious candidate for replacing ITO if the challenges of mass production can be overcome. One promising method for mass production is the chemical conversion route. It most often utilizes a harsh oxidization of graphite based on the Hummers method into graphite oxide (GO),8 with subsequent reduction/deoxygenation into a graphene-like material termed chemically converted graphene (CCG) or reduced graphene oxide (rGO).9 A wide range of reduction procedures have already been proposed to this end,10−20 of which the majority employ a one- or two-step conversion scheme, such as (1) a chemical reduction step, with © 2013 American Chemical Society
Received: May 15, 2013 Revised: November 29, 2013 Published: December 2, 2013 4839
dx.doi.org/10.1021/cm4015942 | Chem. Mater. 2013, 25, 4839−4848
Chemistry of Materials
Article
Scheme 1. Deposition and Reduction Schemea
a After GO synthesis and purification, the as prepared mGO films were deposited on Si/SiO2 wafers by LB transfer. After LB deposition, the mGO films were reduced with excess hydrazine, yielding rGO1. The films were then treated with sulfuric acid to obtain rGO2. To obtain the final product rGO3 the films were annealed at high temperature in a reducing atmosphere. As reference samples, both mGO, LB, and rGO1 films were annealed at similar conditions to give a-mGO and a-rGO1, respectively.
temperature (150/550 °C) annealing was developed by Zhu and co-workers for a composite thin film with poly(diallyldimethylammonium chloride) as counter electrolyte.25 The oxygen content for these films was, however, not reported, and their samples were instead characterized by electrical measurements and Raman spectroscopy. While most of the deoxygenation of GO occurs below 500 °C,14,26 annealing to 1,000 °C is necessary to decompose double-bonded oxygen.14 Very recently it has been shown that a full conversion of thin films of GO and hydrazine-reduced GO can also be achieved at extremely high temperatures around 2,000 and 1,500 °C, respectively.27,28 It was also shown that at temperatures in excess of 2,400 °C, repair of the structural and topological defects occurred. Besides complete removal of oxygen, a major problem common to the chemical conversion methods has been to keep the GO from aggregating during chemical conversion without the use of stabilizers, which could be hard to get rid of after conversion.9 Control of pH to tune the stability of the rGO has been proposed29 but still fails to keep the sheets from forming intrasheet wrinkles and aggregates upon extensive reduction. One way to circumvent aggregation and wrinkling could be to immobilize the monolayer GO (mGO) sheets on a surface prior to the reduction/deoxygenation step. A relatively wellcontrolled way of doing this is by LB deposition where mosaic films of mGO are assembled and transferred to cm2-sized wafers, as first demonstrated by Cote et al.17,30 By controlling the compression on the LB, the packing densities of the mGO sheets can be controlled to produce films of diluted, close packed, or overlapping flakes. Fabrication of reduced LB films has been reported on several occasions, where the strategy has been to use either expanded graphite,31 or post deposition reduction with a one- or two-step conversion procedure.17,32−34 Here we report a post deposition, three-step conversion procedure for mGO LB films involving (1) reduction with hydrazine monohydrate in dimethylformamide (DMF), (2) deoxygenation with concentrated sulfuric acid, and (3) high temperature annealing, as outlined in Scheme 1. This procedure is applied to immobilized mGO sheets both in LB films of diluted single sheets and in dense coherent LB films of overlapping sheets. XPS revealed an oxygen content close to zero (
