Apparent Roughness as Indicator of (Local) Deoxygenation of

Aug 11, 2014 - Detailed characterization of graphene oxide (GO) and its reduced forms continues to be a challenge. We have employed scanning tunneling...
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Apparent Roughness as Indicator of (Local) Deoxygenation of Graphene Oxide Duncan den Boer,* Jonathan G. Weis, Carlos A. Zuniga, Stefanie A. Sydlik, and Timothy M. Swager* Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States S Supporting Information *

ABSTRACT: Detailed characterization of graphene oxide (GO) and its reduced forms continues to be a challenge. We have employed scanning tunneling microscopy (STM) to examine GO samples with varying degrees of deoxygenation via controlled chemical reduction. Analysis of the roughness of the apparent height in STM topography measurements, i.e. the “apparent roughness”, revealed a correlation between increasing deoxygenation and decreasing apparent roughness. This analysis can therefore be a useful supplement to the techniques currently available for the study of GO and related materials. The presence of a high electric field underneath the STM tip can locally induce a reaction on the GO basal plane that leads to local deoxygenation, and the restoration of the sp2 hybridization of the carbons promotes increased planarity. These findings are in line with the apparent roughness values found for GO at varying levels of chemical reduction and illustrates the value of having a tool to gain structural/chemical insight on a local scale. This is the first example of employing an STM tip to locally reduce GO to reduced GO (rGO) and partially reduced GO (prGO) without locally destroying the graphene sample. Local manipulation on the nanoscale has utility for graphene nanoelectronics, and analysis employing the apparent roughness is an additional tool for the study of graphene oxide and related basal plane chemistry.

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INTRODUCTION The discovery of isolated single layer graphene a decade ago1 has resulted in a flurry of research activity. Graphene’s applications are foreseen within a wide variety of fields including electronics, energy storage, chemical sensing, and biological applications.2−4 For this material to become widely implemented as an industrial technology, methods for its mass production must be understood. A promising approach for applications that can tolerate some residual defects makes use of thermally or chemically reducing dispersions of graphene oxide (GO) to create electronically active materials.5−8 GO is readily synthesized by oxidation of graphite and subsequently exfoliated to form dispersions of single sheets of GO in water or other organic solvents.9 Beyond imparting dispersibility, the oxygen-containing moieties in the material can also act as chemically reactive handles that can be exploited for installing new functional groups8 that can tune the electrical and material properties.10,11 Proper descriptions of the structure of GO are under debate;5,7,8,12,13 however, it is undeniably a chemically complex material with different types and local environments of oxygencontaining functionalities. There are typically large batch-tobatch variations in GO with differences in oxygenation levels. One of the main challenges for elucidating the exact structure of GO has been the difficulty of its characterization.8,10,14,15 It is therefore important to further explore characterization tools for © XXXX American Chemical Society

the study of these materials. In this study, we explored the use of scanning tunneling microscopy (STM) as a technique for distinguishing between GO samples with different levels of oxygenation from either controlled reductions of conventionally synthesized GO or from the synthesis of a less oxidized GO (loGO). The relatively planar nature of graphene oxide and its derivatives is suitable for characterization by scanning probe microscopy (SPM). The high spatial resolution of SPM techniques provides useful insight into the nanoscale structure, which is essential for an in-depth understanding of the processes occurring during (de)oxygenation and functionalization. Atomic force microscopy (AFM) is the SPM technique most widely used for this type of research that in addition to visualization allows determination of local friction,16 conductivity,16,17 and differences in interaction of the sample with the cantilever (visible in the phase image).9,18 From these methods AFM can be used to infer information on the chemical composition of the material under study. However, in most cases AFM analysis is limited to imaging and measuring the apparent height features. In contrast STM has been employed only sparingly for the analysis of GO and its deoxygenated Received: June 13, 2014 Revised: July 25, 2014

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forms. GO,18−23 reduced graphene oxide (rGO),22,23 and partially reduced graphene oxide (prGO)24 have been visualized by STM at different levels of detail, in different environments (ultrahigh vacuum or ambient conditions and with different surfaces underneath). In general, these STM measurements vary substantially. It is difficult to draw conclusions from the often subtle differences in STM topography images of GO-related materials. In this context studies of more than one or two different types of GO or (p)rGO need to be compared. Although there have been some efforts in this direction,24 batch-to-batch reproducibility10,13,25 and the present state of STM topography characterization falls short of enabling different GO derivatives to be distinguished. To extract reliable and meaningful data from the samples, we introduce herein the analysis of the “apparent roughness” and use STM as a characterization tool for GO and its reduced forms. STM has proven a valuable tool for determining the roughness of surfaces.26−28 In most cases, the surfaces have been homogeneous with uniform electronic characteristics. However, for surfaces with electronic heterogeneity the topographic STM images are a convolution of geometrical features of the surface and its local electronic properties.29,30 As a result for the heterogeneous surfaces of GO and its derivatives, we measure the “apparent roughness” and emphasize its dependence on both the local electronic and geometrical properties. The root-mean-square roughness, Rq, as defined by Rq = ((1/L)∫ L0 Z2(x)dx)1/2 represents the standard deviation of the distribution of surface heights.31 Recent calculations and chemical intuition suggest that graphene becomes less planar with increasing oxidation, as a result of the conversion of sp2 to sp3 carbon atoms.32−34 This roughening of the surface upon oxidation (as compared to a pristine graphene layer) has been observed experimentally in a qualitative manner.9,22,35,36 Calculations34 and experiments36 further suggest that upon deoxygenation of the graphene oxide, the surface roughness is reduced. GO in its highly oxidized form is insulating and with reduction to rGO partially recovers graphene-like conductive properties.5 Differences in planarity and conductivities of GO, prGO, and rGO have an effect on the apparent roughness, and this suggests that apparent roughness can provide valuable information about the material including its level of deoxygenation. In addition to characterization, SPM techniques permit the manipulation of GO and rGO at the nanoscale, locally changing the chemical structure of the material. Such local manipulation opens up applications in graphene nanoelectronics.16 In recent years, AFM has been employed for the local reduction of GO by using a heated tip,16 an applied bias voltage,17,37 or a Ptcovered tip in a hydrogen atmosphere.38 Pristine graphene has been locally oxidized by an applied bias voltage on the AFM tip,39,40 and AFM has also been employed for local lithography by removing parts of flakes.41 Although STM has been applied for the local lithography of graphene42,43 and GO,23 it has not been used for the study of the local reduction of GO to rGO or prGO. For reference samples we analyze the apparent roughness of macroscopically chemically reduced GO flakes, with different levels of deoxygenation. Comparisons can then be made with locally manipulated GO flakes obtained by direct reduction from GO to prGO or rGO by STM. We determined that inducing the reduction by a negative bias voltage leads to a

partial return to planarity of the GO, the extent of which is controlled by the bias voltage. This demonstration illustrates the possibility to use the STM for local manipulation and local analysis of a GO surface.

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EXPERIMENTAL SECTION

AFM and STM images were obtained with an Agilent 5100. AFM was performed in ACAFM tapping mode using silicon tips with a force constant of 20−80 N/m, STM in the constant current mode with mechanically cut tips from Pt/Ir wires (80:20). The graphite used as the sample surface was HOPG ZYB, NT-MDT and was freshly cleaved before each experiment. Measurements were performed at the graphite/air interface under ambient conditions. V bias values mentioned in the manuscript refer to the surface, with respect to the STM tip. X-ray photoelectron spectroscopy (XPS) was performed on a Versaprobe II X-ray photoelectron spectrometer from Physical Electronics with a monochromated Al Kα X-ray source (1486.6 eV) and operated at a base pressure of 1 × 10−9 Torr during the XPS analysis. The scans were made with a 200 μm size beam with a take off angle of 45° and a total power of 45.7 W. The XPS spectra were analyzed, and atomic peaks were integrated using CasaXPS software to determine the relative atomic percentages of carbon and oxygen present in the samples. All reagents were purchased from Sigma-Aldrich unless otherwise noted. Graphene oxide (GO) and less oxidized graphene oxide (loGO) were synthesized via a modified Hummer’s method,44 and a detailed description can be found in the Supporting Information. Graphitic samples were suspended in water (loGO, GO, prGO2h, prGO4h) or N-methyl-2-pyrrolidone (NMP) (rGO) at a concentration of 0.1 mg/mL and sonicated for 1−10 min using a bath sonicator. A volume of 40−100 μL of the resulting dispersion was then dropcast onto a freshly cleaved HOPG surface, and the solvent was evaporated in a vacuum desiccator before imaging with AFM and STM.

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RESULTS AND DISCUSSION Synthesis of Materials. Graphene oxide (GO) was synthesized using a modified Hummer’s method.44 In order to examine the effect of deoxygenation on GO by AFM and STM, two partially reduced GOs (prGO2h and prGO4h) and a heavily reduced GO (rGO) were prepared by reduction of the prepared GO with l-ascorbic acid.45 We also examined a less oxidized graphene oxide (loGO) made from a 1:1 ratio of oxidant to graphite (versus the conventional 3:1 ratio). The materials prepared were examined by XPS to determine the relative atomic percentages of carbon and oxygen (see Figure S5). The differences in the oxygen present are summarized in Table 1 with conventionally prepared GO possessing the Table 1. XPS Relative Atomic Percentages of O 1s and C 1s loGO GO prGO2h prGO4h rGO

atomic percent (O 1s)

atomic percent (C 1s)

O/C

23.9 29.0 24.0 22.0 16.7

76.1 71.0 76.0 77.9 83.3

0.31 0.41 0.32 0.28 0.20

highest oxygen content and loGO displaying modestly lower oxygen content. The partial reductions of GO at 2 and 4 h showed the oxygen content to be 5 and 7% lower than GO. The fully reduced GO possessed the lowest oxygen content with a 12% reduction in the oxygen content, leading to more than 50% decrease of the O/C value. AFM Studies. A drop of solution containing the graphene materials was applied to a freshly cleaved highly oriented B

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Figure 1. AFM topography images of graphene oxide flakes at varying degrees of deoxygenation at the graphite/air interface after deposition from solution. Top: topography images, depicted z scale bars are in nm; center: phase image; bottom: cross-section, indicated by the dashed line in the corresponding phase image. a) Less oxidized graphene oxide (loGO), deposited from H2O. b) Graphene oxide (GO) deposited from H2O. c) Partially reduced graphene oxide, after 2 h of reduction (prGO2h) deposited from H2O. d) Partially reduced graphene oxide, after 4 h of reduction (prGO4h) deposited from H2O. e) Fully reduced graphene oxide (rGO) deposited from NMP.

Figure 2. STM topography images of graphene oxide flakes at varying degrees of deoxygenation at the graphite/air interface. Depicted z scale bars are in nm. Vbias = +1 V, Iset = 2−5 pA. a) loGO from H2O. b) GO from H2O. c) prGO2h from H2O. d) prGO4h from H2O. e) rGO from NMP.

analysis of at least 100 flakes per compound). This trend suggests reductive fragmentation of the flakes. Attempts to determine the roughness Rq by AFM (see the SI) were unsuccessful with the analysis giving an Rq ∼ 0.1−0.15 nm, implying no difference. This lack of sensitivity to roughness is a result of the large (5 nm radius) AFM tip.46,47 STM with its much smaller tips has an advantage with respect to spatial resolution. It has been shown that GO displays a larger phase contrast with the underlying graphite in AFM measurements,9,18 relative to that observed for rGO. We also observed this effect, and GO flakes were easily distinguished from the graphite background in the phase measurements. For loGO, GO, and prGO2h, the phase difference is about 3−4° with respect to the underlying graphite (Figure 1a,b,c), in accord with values of 2.5° and 5.5° previously observed for GO on graphite,9,18 and ±1° for prGO4h and rGO (Figure 1d and

pyrolytic graphite (HOPG) surface, which displays very low roughness. The high affinity of the GO π−π stacking to the graphite affixes the samples. AFM measurements (Figure 1) were used to determine the density of the flakes on the surface prior to the STM measurements. A sufficiently high coverage level of graphene flakes is necessary as the STM has a limited working scan range, and in this context the insolubility of rGO in water proved problematic and required prolonged sonication times to create low concentration dispersions of rGO in NMP. The average flake sizes observed were also smaller than for the less reduced compounds, although this generalization is complicated by a large variation in flake sizes with a standard deviation of about 100% (for rGO the average flake size was ∼1 × 104 nm2, while for prGO4h this was ∼1.6 × 104 nm2, for prGO2h ∼2 × 104 nm2, loGO ∼4 × 104 nm2, and for GO 1.5 × 105 nm2, based on C

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Figure 3. Apparent RMS roughness of graphene oxide flakes at different degrees of deoxygenation on graphite obtained from STM measurements. For every bar n = 9−11 flakes, error bars represent the standard deviation; measurements obtained with at least 2 STM tips. Vbias = +1 V, Iset = 2−5 pA. Indicated on the right is the apparent roughness that was found for the local manipulation experiments (see text and Figure 4), in which parts of the surface of loGO were scanned at the indicated voltage.

are small, and it is challenging to distinguish the results of loGO (Figure 2a), GO (Figure 2b), and prGO2h (Figure 2c). The more reduced materials, prGO4h (Figure 2d) and rGO (Figure 2e), appear different from the more heavily oxidized flakes and have similarly smooth surfaces. The observed difference between prGO2h and prGO4h is surprising, as there is only a small difference present in terms of the O/C ratio (0.31 vs 0.28). STM images of prGO2h are similar to the moderately reduced flakes observed in literature,24 but images of more fully reduced prGO4h and rGO appear (by visual examination) to be smoother than the published heavily reduced materials.22,24 To establish a more precise correlation between planarity and deoxygenation, we analyzed the apparent roughness of the different types of flakes (Figure 3). The apparent roughness was determined at the same voltage for all the flakes, +1 V, at a current of several pA, for n = ∼10 flakes, and was measured with at least two different STM tips. There were no differences observed between the tips. To reduce the introduction of artifacts, all measurements were subjected to the same type of background correction (see the SI). Areas were selected from STM images with the same pixels/nm2. To decouple the longrange waviness from the short-range roughness,49 the measurement is subjected to an ISO 16610-61 L-Filter. We measured several GO flakes for long periods of time (n = 3, for 3, 16, and 20 h) and compared the apparent roughness of the same area before and after this time interval. We found that the difference remained below 5%, which is well within the observed flake to flake variation of ∼10%. Within a flake this difference remained below 6% (determined by comparison of four different sections on three flakes). There seemed to be a dependence on voltage, with negative voltages (