Controlled Spacing of Few-Layer Graphene Sheets Using Molecular

Article Cite This: ACS Appl. Nano Mater. 2018, 1, 1420−1429

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Controlled Spacing of Few-Layer Graphene Sheets Using Molecular Spacers: Capacitance That Scales with Sheet Number Anna K. Farquhar, Paula A. Brooksby,* and Alison J. Downard* MacDiarmid Institute of Advanced Materials and Nanotechnology, School of Physical and Chemical Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

ACS Appl. Nano Mater. 2018.1:1420-1429. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 08/28/18. For personal use only.

S Supporting Information *

ABSTRACT: Preventing graphene sheet aggregation while retaining full accessibility to the total graphene surface area is key to optimizing the performance of graphene supercapacitors. Spacer species can be added to graphene assemblies to prevent aggregation, but typical methodologies do not allow accurate assessment of the extent of sheet separation nor whether spacing groups block some of the surface area. In this work we have grafted sub-10 nm films of aryl spacer groups to chemical vapor deposition-grown few layer graphene (FLG) sheets using aryldiazonium salts. Using a layer-by-layer strategy that relies on individually handling each FLG sheet, 3-sheet stacks were prepared and electrochemically interrogated. By comparing the differential and integral capacitances of modified and unmodified single FLG sheets and 3-sheet stacks, we show that in the 3sheet stacks of modified FLG, the grafted spacer groups fully separate the FLG sheets and allow complete double layer formation at all FLG-solution interfaces. The stacks of modified FLG show no decrease in double layer capacitance over 20 000 charge− discharge cycles, indicating that full separation of the FLG sheets is maintained and confirming that these grafted spacer layers offer a practical solution to the problems of graphene aggregation. KEYWORDS: supercapacitor, carbon, electrochemical impedance spectroscopy, charge−discharge, differential, integral, aryldiazonium ion, grafting

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INTRODUCTION Graphene has great potential for a wide range of applications due to its impressive array of properties, including its high surface area:mass and volume ratios, and high electrical conductivity.1−4 However, exploitation of the high surface area of graphene in workable devices is currently limited because the strong van der Waals interactions between the graphene sheets lead to aggregation or restacking of the sheets during assembly and processing.5−7 This behavior is a particular disadvantage when constructing supercapacitors from graphene because the charge stored directly scales with the area of the surface that is accessible to electrolyte ions. Therefore, prevention of sheet aggregation is a key factor in the fabrication of supercapacitor devices from graphene.7,8 Several strategies have been implemented to prevent the aggregation of graphene during both electrode assembly and subsequent potential cycling. These include the use of spacers, template assisted growth giving three-dimensional porous graphene materials, and crumpling.9 The spacer approach © 2018 American Chemical Society

aims to introduce nanoscale entities between the graphene sheets.5,7,10−13 For example, when a dispersion of exfoliated graphene was decorated with Pt nanoparticles in the low nanometer range, the dried material showed an increase in the Brunauer−Emmett−Teller (BET) surface area from 44 to 862 m2 g−1, with a corresponding increase in capacitance from 14 to 269 F g−1.12 Similarly, introduction of 10 nm Au nanoparticles was found to give a more than 50× increase in the capacitance of electrochemically reduced graphene oxide (ErGO) (based on the mass of ErGO).14 Carbon nanotubes (CNTs) are often used as spacers in graphene composite materials. In one example, the simple approach of mixing, drop casting, and drying a GO and CNT dispersion gave films in which the graphene sheets were proposed to be separated by CNTs.10 After electrochemical reduction of the films, charge−discharge Received: February 19, 2018 Accepted: February 26, 2018 Published: February 26, 2018 1420

DOI: 10.1021/acsanm.8b00280 ACS Appl. Nano Mater. 2018, 1, 1420−1429

Article

ACS Applied Nano Materials

carboxyphenyl (CP). CP groups were grafted directly from the corresponding aryldiazonium salt, whereas AP groups were generated by reduction of a nitrophenyl (NP) layer which was also grafted from the aryldiazonium salt.

measurements revealed that although the gravimetric capacitance measured at low charge−discharge rates was not increased by the addition of CNTs, the capacitance at higher rates was significantly increased. These findings suggest that ion transport to the graphene surface was improved in the presence of CNTs but that the total available surface area did not increase.10 In a later study following similar procedures for preparation of ErGO−CNT composites, it was shown that the capacitance was the average of gravimetric capacitances of the rGO and CNTs.15 In another simple, bioinspired approach, water was trapped between rGO layers simply by depositing a film from a hydrated rGO dispersion. The water separated the rGO sheets leading to an increase in capacitance,5 however the excellent capacitive properties were not maintained after drying the film. Recently, in very elegant work by Lee and co-workers, rGO sheets were separated by molecular spacers of controlled length through covalent attachment of mono-, bi- and triphenyl groups derived from the corresponding aryldiazonium ions.13 The largest capacitances were obtained for the biphenyl spacer which gave an intersheet separation of approximately 0.7 nm. Although this study achieved impressive control of the intersheet separation, a drawback of all solution-based methods is that it is not possible to finely control the number of graphene sheets in the final electrode material. As a consequence it is impossible to accurately determine the fraction of sheets that are effectively separated, and the impact of the spacer groups on the per-sheet capacitance. Achieving maximum capacitance for a given graphene material relies on fully preventing restacking of the graphene sheets while not impeding double layer formation at the graphene surfaces. In previous work we demonstrated that under selected reaction conditions, covalently bonded aryl films grafted from aryldiazonium salts do not diminish the total capacitance of a single sheet of few layer graphene (FLG) grown by chemical vapor deposition (CVD).16 We also showed that when mounted in our electrochemical cells with the graphene plane parallel to the support, there is unobstructed access of electrolyte solution to both sides of the modified (and unmodified) FLG sheets,17 an observation we attributed to porosity arising from the defective nature of the sheets. This suggests that the modified FLG sheets should be suitable building blocks for constructing multilayer stacks of parallel separated graphene sheets with capacitance that scales with the number of sheets. The goal of the present work was to demonstrate that molecular layers grafted to FLG from aryldiazonium salts can fully prevent restacking of FLG under conditions where the per-sheet capacitance is not diminished by the spacer groups or the stacking arrangement. To achieve this goal we have utilized macroscale (1 cm × 1 cm) 3−4 layer graphene sheets and have adopted a layer-by-layer (LBL) assembly procedure18,19 in which each sheet of FLG is handled individually allowing us to precisely control the number of stacked sheets as well as their intersheet separation. Our approach has enabled us to compare the capacitance of stacks of 3 modified FLG sheets with the capacitance of a single FLG modified sheet, and with the capacitance of a 3-sheet stack of unmodified FLG. (3-sheet stacks were chosen as a compromise between being able to demonstrate the effects of stacking FLG sheets while limiting the fabrication time.) We investigated both the effect of spacer layer thickness and also the chemical nature of spacer groups by modifying FLG with thin films of aminophenyl (AP) and

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EXPERIMENTAL METHODS

Chemicals. All chemicals were purchased from commercial sources and used as received, and all solvents were HPLC grade. The gases used for preparation of graphene were purchased from BOC New Zealand. Aqueous solutions were prepared using Milli-Q water (resistivity >18 MΩ cm). Nitrobenzenediazonium tetrafluoroborate (NBD) and carboxybenzenediazonium tetrafluoroborate (CBD) were prepared using a standard procedure.20−22 Chemical Vapor Deposition (CVD) Growth of FLG and Removal from Cu Foil. Copper foil (99.999%, 25 μm thick, Alfa Aesar) was cleaned successively in dilute nitric acid, Milli-Q water and isopropanol, and dried under a stream of nitrogen. The foil was cut into 1 cm × 1 cm pieces and placed in a furnace within a quartz tube. The furnace was heated to 100 °C with 50 sccm H2 and 400 sccm Ar for 30 min, after which the temperature was increased to 1050 °C for 30 min and 1060 °C for 30 min under 1000 sccm Ar and 300 sccm H2. The Ar flow rate was increased to 1500 sccm, the H2 flow rate decreased to 200 sccm, and methane introduced at 5 sccm for 5 min at 1060 °C. The furnace was cooled for 1 h to 600 °C under 1500 sccm Ar and 50 sccm H2 only, then the FLG/Cu samples were withdrawn from the heating zone and cooled rapidly to room temperature. To remove FLG from its Cu support, the FLG/Cu sample was floated on ammonium persulfate solution (0.5 M) for 15 min to remove the FLG from one side.23 This FLG was discarded. The FLG/ Cu coupon was refloated, Cu side down, on fresh ammonium persulfate solution (0.5 M) in a watch glass until the Cu was etched completely, leaving a free-floating FLG sheet. FLG sheets comprised 3−4 layers of graphene as determined by transferring free-floating FLG to a quartz slide and measuring the decrease in transparency between 600 and 800 nm (Supporting Information, Figure S1).24 Graphene quality, and successful modification was assessed using Raman spectroscopy, as described in previous work.17 FLG Modification. The protocol for modifying FLG using aryldiazonium ion chemistry has been described previously.17 Grafted layers can range from submonolayer to multilayer, depending on the reaction conditions; Scheme S1A, Supporting Information, depicts the generally accepted multilayer film structure. Nitrophenyl (NP) groups were grafted to FLG before removing the FLG from the Cu coupon. The FLG/Cu coupon was floated on an aqueous solution of NBD for the specified time period in the dark, then rinsed five times with MilliQ water. Unless stated otherwise, the modification solution was 20 mM NBD and the modification time was 7 h. The modified FLG was removed from the Cu foil, as described above. Figure S2, Supporting Information, shows an infrared (IR) spectrum of the NP-modified material. The surface concentration of electroactive immobilized NP groups was obtained by transferring the modified FLG sheet to a highly ordered pyrolytic graphite (HOPG) electrode and reducing the grafted film as described previously.17 The charge associated with reduction of NP groups and oxidation of hydroxylaminophenyl groups was used to estimate the surface concentration of NP groups.17 Scheme S1B shows the stoichiometry of the redox reactions. The geometric surface area was assumed for this calculation. Carboxyphenyl (CP) groups were grafted to FLG after its removal from the Cu coupon. Free-floating FLG was transferred to a 20 mM aqueous solution of CBD for 7 h, in the dark. The FLG was then washed five times with water using a pipet to remove and replace the water. Figures S3 and S4, Supporting Information, show an IR spectrum and atomic force microscopy (AFM) images of the modified material. The AFM images confirm that CP groups are grafted to only one side (the solution side) of the FLG. In previous work we demonstrated that the grafting procedure used for NP groups also results in one-side modification of FLG.16,17 Note that the on-coupon modification procedure used to prepare NP-modified FLG could not 1421


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Epoxy/Au Electrode Preparation. For electrochemistry, 1 cm × 1 cm Au (50 nm)/Ti (50 nm)/Si substrates were prepared using standard lithographic methods on a Si(100) wafer. A drop (∼0.1 mL, diameter ∼3 mm) of epoxy resin (EPO-TEK 301, two-part resin), was placed on the center of each wafer and cured. Electrochemistry. All electrochemical measurements were performed using an Eco Chemie Autolab PGSTAT302N potentiostat running Nova software. FLG working electrodes were fabricated by collecting free-floating FLG samples (1- and 3-sheet stacks) atop the drop of cured epoxy on the epoxy/Au substrates, ensuring the FLG edges were in contact with the Au. After washing with methanol and drying at room temperature and 60 °C, a second epoxy layer was used to seal the FLG onto the epoxy and define the approximately circular working electrode area (Figure 1). The geometric working electrode area was determined using ImageJ software. The cell was assembled with the working electrode placed horizontally between an insulated metal base plate and a glass solution cell with a hole in the base (Figure 1). A copper foil was taped to the working electrode to make electrical contact with the graphene and Au support. An O-ring and four springs from plate to the cell sealed the electrolyte solution above the sample. A large area Au wire electrode was used as the counter electrode and a saturated calomel electrode (SCE), housed in a Luggin capillary, was used as the reference electrode. The electrolyte solution was sparged with N2 for 15 min prior to all measurements. Before and after all electrochemical impedance spectroscopy (EIS) measurements, cyclic voltammograms (CVs) were obtained over the potential range −0.6 to 0.3 V at 200 mV s−1 to confirm that the electrodes were unchanged by EIS measurements (see Supporting Information Figures S5−S7). EIS data were collected using a perturbation amplitude of 10 mV at 50 frequencies, from 0.1 MHz to 0.1 Hz, between −0.5 and 0.3 V in 50 mV steps, starting at −0.5 V. The DC potential was applied for 2 min before beginning each measurement to allow the system to achieve steady state. All EIS measurements were started at the most negative potential and stepped to more positive potentials. In the absence of any Faradaic processes, the differential capacitance at each potential can be calculated based on the following equation:

be used for CP modification. After grafting CP groups to FLG on Cu, the Cu coupon was found to sink in the etchant solution, damaging the FLG sheet. We attribute this behavior to the hydrophilicity of the CP layer. Layer-by-Layer Assembly of FLG Stacks. Stacks of FLG with AP spacer groups were prepared from FLG that had been modified using NBD. The NP-modified FLG/Cu coupon was taped to a glass microscope slide and a second, free-floating NP-modified FLG sheet was collected onto the FLG/Cu surface by lifting the microscope slide from below the floating FLG. This substrate was dried for 1 h at room temperature followed by 30 min at 60 °C. Another modified FLG sheet was collected onto the FLG/FLG/Cu substrate in the same manner, giving a 3-sheet stack of NP-modified FLG on Cu. The Cu foil was removed as described above, giving a free-floating 3-sheet stack. This protocol was repeated with unmodified FLG, giving a 3sheet FLG reference sample. The NP-modified and unmodified FLG stacks, and single sheet FLG samples were transferred onto an epoxy/ Au electrode (see below) by lifting the substrate from below the freefloating FLG and drying for 1 h at room temperature followed by 30 min at 60 °C. In the final step NP groups were converted to (mainly) AP groups by electrochemically reducing NP-modified FLG (stacks and single sheets) using 2 repeat cyclic voltammetric (CV) scans at 200 mV s−1 in 1 M HClO4 over the potential range 0.6 to −1.0 V. Scheme S1B, Supporting Information, illustrates this reduction step. Figure 1, cartoons A and B, show the arrangement of the AP-modified FLG on the electrode.

C=

1 2πf (− Z″)

where C is the differential capacitance in Farads, f is the frequency (115 Hz for this work), and Z′′ is the imaginary component of the impedance.16,25,26 CVs for capacitance measurements were recorded between 0 and 0.4 V, at scan rates of 200, 100, 50, 20, 10, 5, and 2 mV s−1, in order of decreasing scan rate. Galvanostatic charge−discharge measurements (CD) were obtained after the CVs described above. For CD experiments, electrodes were charged from 0 to 0.4 V, at current densities of 6.25, 10.0, 12.5, 25.0, and 50.0 μA cm−2, in order of increasing current density, with 15 CD cycles at each current density. Experiments investigating the stability of modified one-sheet and 3-sheet stacks were performed at a current density of 10 μA cm−2 after the set of CD measurements described above. The capacitance was calculated based on the discharge portion of the plot using the equation:

Figure 1. FLG working electrode formats and arrangement of the 3electrode cell. (A, B) AP-modified FLG; (C, D) CP-modified FLG. (A, C) single FLG sheet; (B, D) 3-sheet stacks. Note: AP groups were prepared by reduction of NP groups. For CP-modified FLG stacks, a free-floating CP-modified FLG sheet was collected onto an epoxy/Au electrode (see below) by lifting the substrate from below the floating sheet and drying as above. A second and third free-floating CP-modified FLG sheet were collected onto the electrode in the same manner, giving a 3-sheet stack of CPmodified FLG on the epoxy/Au electrode. A CP-modified single sheet of FLG was collected on the epoxy/Au electrode by the same method. Figure 1, cartoons C and D, show the arrangement of the CP-modified FLG on the electrode. As depicted in Figure 1, the assembly procedures described above result in two different orientations of the modified FLG sheets on the substrate (i.e., with the modifier layers facing upward (NP) or downward (CP)). This is a consequence of the different procedures used for grafting NP and CP groups to the FLG (on-Cu and on freefloating FLG, respectively). However, as described in the following, the orientation of the FLG appeared to have no influence on the behavior of the single FLG layers or stacks.

C=

J ΔV /t D

where C is the integral capacitance in F cm−2, J is the current density in A cm−2, ΔV is the potential window in V, and tD is the discharge time in seconds. For all calculations, the working electrode area was assumed to be the geometric area. Stated uncertainties and error bars are the standard deviation for n samples (for n > 2), or indicate the range of values if only two samples were measured. 1422


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RESULTS AND DISCUSSION The differential capacitances at selected potentials of a single sheet of 3−4 layer FLG and a 3-sheet stack of FLG with no molecular spacers separating the sheets (3FLG) were obtained from EIS measurements and are shown in Figure 2. In previous

of graphene layers increases from 3 to 4 (for FLG) to 9−12 (for 3FLG) in not unexpected. Ruoff et al.26 report that increasing the number of layers decreases the minimum capacitance and the slope of the capacitance versus voltage plot, as the values trend toward those commonly reported for HOPG. For the 3FLG samples, the minimum capacitance and the slopes of the plot are similar to those reported for HOPG (2−3 μF cm−2 and 1 to 2.5 μF cm−2 V−1, respectively), with the left arm showing a smaller slope as typically observed for HOPG.29,30 Evidently, stacking 3 unmodified FLG sheets results in an HOPG-like electrode indicating that there is insignificant separation between the sheets. To investigate the effect of spacer groups on the capacitance of FLG stacks, the differential capacitances of single sheets and 3-sheet stacks of modified FLG were investigated by EIS measurements under the same conditions as for the unmodified materials. Nitrophenyl (NP) and carboxyphenyl (CP) groups were grafted to one side of FLG sheets under the conditions listed in Table 1. These conditions were expected to yield NP layers ranging from submonolayers to multilayers.17 NP was selected as a modifier because its electroactivity allows the surface concentration of grafted groups to be experimentally measured. Reduction of NP groups produces AP groups plus a small proportion of hydroxylaminophenyl groups (Supporting Information, Scheme S1B). The latter are electroactive over the potential window of interest for capacitance measurements and add a significant pseudocapacitance (see below). However, they have poor stability to redox cycling (see Figure S10) giving a changing total capacitance over the initial few electrochemical experiments; this adds complication to analysis. CP groups were chosen as a second modifier to investigate whether the chemical nature or charge of the spacer layer (positively charged (AP) and neutral (CP)) influences the behavior of the stacks and to examine the performance of stacks without a significant pseudocapacitive component. The surface concentrations of NP groups (Table 1) were estimated from electrochemical measurements and are assumed to correspond to the surface concentration of AP groups after reduction of NP-modified FLG (Supporting Information, Scheme S1B).17,31,32 Average film thicknesses were estimated using the experimentally established relationship for NP films: thickness (nm) = surface concentration (mol cm−2)/((3.25 ± 0.5) × 10−10 mol cm−2 nm−1).21 CP groups are electroinactive and hence their surface concentration could not be measured. Using freshly grafted NP- and CP-modified FLG, 3-sheet stacks were assembled on epoxy/Au electrodes using our LBL procedure. Prior to EIS measurements on NP-modified FLG (stacks and single sheets), NP groups were electrochemically

Figure 2. Differential capacitance at selected applied potentials derived from EIS in 1 M HClO4, for (blue) FLG (n = 6) and (pink) 3FLG (n = 2).

work, we established that in the working electrode configurations used here, the electrolyte solution can access both sides of the FLG sheet, presumably via defect and grain boundary regions,17 thus the electrical double layer is expected to form on both sides of the FLG. The plot for the single FLG sheet shows a U-shaped dependence in agreement with literature reports.16,25−27 The large uncertainty associated with the capacitance values arises from measurement of FLG samples prepared in different positions in the furnace and in different runs, however for each FLG sample tested, a U-shaped plot was obtained. There is an average minimum capacitance of 4.2 ± 0.9 μF cm−2, that sits at the Dirac point.28 On either side of this minimum, the capacitance increases with an average slope of 3.1 ± 1.1 and 2.4 ± 1.3 μF cm−2 V−1, for the right and left arms, respectively. This increase in capacitance on either side of the Dirac point is caused by an increase in the hole and electron doping.26 The minimum value and slope agrees with work by Ruoff et al. for 3−4 layer graphene.26 In comparison, the 3FLG stack prepared using our LBL strategy displays a lower minimum capacitance (2.8 μF cm−2) and smaller slope of the right and left arms (2.4 and 0.7 μF cm−2 V −1, respectively). Although the uncertainty in the capacitance values makes the significance of these differences unclear, a decrease in the capacitance as the number

Table 1. Grafting Conditions (Solution Concentration of Aryldiazonium Ion and Grafting Time), Surface Concentration of Spacer Groups (Γ), Average Spacer Layer Thickness, and Minimum Capacitance Obtained from EIS Measurements, for 1-Sheet Systems and 3-Sheet Stacks conditions for modifying FLG

spacer group

Γa × 10−10/mol cm−2

Unmodified FLG NBD 1 mM, 1 h NBD 20 mM, 7 h NBD 20 mM, 16 h NBD 20 mM, 72 h CBD 20 mM, 7 h

− AP AP AP AP CP

− 1.2 ± 0.4 2.1 ± 0.2 4.7 ± 0.6 19.2 ± 1.3 −

spacer thicknessb/nm − 0.4 0.7 1.5 6.0 −

nm nm nm nm

1-sheet capacitance/μF cm−2

3-sheet capacitance/μF cm−2

± ± ± ± ± ±

2.8 ± 0.9 8.5 ± 0.9 17.9 ± 1.4 18.1 ± 4.3 19.1 ± 0.9 17.2 ± 0.9

4.2 3.9 5.6 4.4 5.8 5.8

0.9 1.1 1.3 0.2 0.2 0.9

a

Measured surface concentrations of NP groups. The same values are assumed for AP groups. bEstimated using the relationship between surface concentration and film thickness for NP layers grafted to pyrolyzed photoresist film.21 1423


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Figure 3. Capacitance versus potential plots derived from EIS at 115 Hz in 1 M HClO4 for (red) FLGAP; (green) 3FLGAP; (orange) FLGCP; (purple) 3FLGCP. The concentration of aryldiazonium ion the grafting solution, and the grafting time were (A) 1 mM, 1 h; (B) 20 mM, 7 h; (C) 20 mM, 16 h; (D) 20 mM, 72 h; (E) 20 mM, 7 h.

reduced to aminophenyl (AP) groups.17,31,32 This material is referred to as “FLGAP”. A CV scan was then obtained between −0.6 and 0.3 V to confirm that the electrode had no Faradaic processes over that potential window, and to act as a reference point for CVs obtained after EIS measurements (see Supporting Information Figures S5−S7). For CP-modified FLG (FLGCP), only a single CV scan between −0.6 and 0.3 V was recorded prior to obtaining EIS data. Capacitance versus potential plots obtained from EIS measurements are shown in Figure 3 and minimum capacitance values are listed in Table 1. The data in Table 1 confirm, as reported previously, that the surface modification does not significantly change the capacitance of single FLG sheets.16 This is not surprising for the low density film grafted over 1 h from a 1 mM NBD solution. A monolayer of AP groups on a flat surface has a calculated surface concentration of 9.5 × 10−10 mol cm−2 (assuming an elliptical footprint, no rotation and complete coverage of the surface33), and hence a surface concentration of 1.2 × 10−10 mol cm−2 corresponds to less than 15% coverage of the surface. On the other hand, it seems surprising that the multilayer of approximately 6 nm thickness grafted over 72 h from a 20 mM solution of NBD does not decrease the capacitance. We assume that the low packing density of multilayer films grafted from aryldiazonium salts21 accounts for this observation. When FLG sheets modified with AP- or CP- groups are assembled into 3-layer stacks, the capacitance increases significantly. For the lowest surface concentration of AP groups which gives an average film thickness of less than a monolayer, the 3-sheet stack has a minimum capacitance approximately 2× that of its 1-sheet counterpart. This suggests that this surface modification is insufficient to separate the FLG sheets over their full extent, or that although close restacking of the FLG is

prevented, the intersheet spacing is insufficient to allow unimpeded electrolyte access to the inner FLG surfaces. For modification conditions that give thicker AP and CP films, the 3-sheet stacks all exhibit minimum capacitances at least 3× those of the corresponding modified single FLG sheets. There is no significant difference in the performance of the stacks for spacer films of estimated thickness 0.7, 1.5, and 6.0 nm, nor between the stacks with AP and CP groups as the spacers. Evidently the charge on the spacer layer (positive for the protonated AP groups and neutral for CP groups in 1 M HClO4) has an insignificant effect on the measured minimum capacitance. Morphological changes to the FLG sheets resulting from the modification may also pay a role in sheet separation however this is unlikely to be a major effect. We have previously used AFM to compare the morphology of unmodified and modified FLG sheets after transfer to a solid substrate and found no significant differences.16,17 The data in Figure 3 and Table 1 clearly demonstrate that modification of FLG using aryldiazonium salts is able to effectively separate FLG sheets and at the same time allows the FLG surfaces to be fully accessible to electrolyte ions. On the other hand, the data do not reveal the peak in capacitance previously observed experimentally13 and modeled theoretically34,35 for graphene materials with specific sub-1 nm sheet separations. It was not feasible to explore this behavior in the present work. The spacing between modified FLG sheets in the sub-1 nm range is subject to large uncertainty and cannot be reliably controlled in sufficiently small increments. Hence it is possible that a jump in the capacitance of a 3-sheet stack might be achieved at a specific low intersheet spacing, but we were unable to obtain evidence for this behavior. 1424


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for AP- and CP-modified FLG (Figures 5B and 5C, respectively), the CVs of the 3-sheet stacks enclose a much larger area than do the CVs of the single modified FLG sheets confirming that the modifying layers effectively separate the sheets and allow double layer formation. The CVs of FLGAP and 3FLGAP show Faradaic processes near 0.4 V assigned to the hydroxylaminophenyl/nitrosophenyl couple. Hydroxylaminophenyl groups are formed during reduction of NP groups prior to capacitance measurements (Supporting Information, Scheme S1B).31,32,48,49 The CV of 3FLGCP also has a Faradaic component centered near 0.12 V, assumed to arise from surface oxygen functionalities, however this is a small contributor to the total capacitance. It is unclear why the same redox response cannot be observed for the single FLGCP sheet. Figure 5D shows the CD responses of FLG, FLGAP, FLGCP, 3FLGAP, and 3FLGCP assemblies. The triangular shapes of the FLG, FLGCP, and 3FLGCP plots are indicative of electrical double layer capacitance, while the bell-shapes seen for FLGAP and 3FLGAP are consistent with a pseudocapacitive contribution, as expected based on the CV responses. The greater CD times of the modified 3-sheet stacks arise from their greater capacitances. Figures 5E and 5F show the capacitances determined from 20 000 CD cycles versus cycle number for FLG and for the single- and 3-sheet modified FLG assemblies. For both spacer types, the capacitance of the 3-sheet stacks is approximately 3× that for the corresponding single sheet, regardless of cycle number. During the initial few hundred CD cycles there is a significant drop in capacitance for FLGAP and 3FLGAP. Comparison of CVs obtained before and after CD cycling (Supporting Information Figure S10) confirm that this is due to the loss of the pseudocapacitance contribution associated with the hydroxylaminophenyl/nitrosophenyl redox couple. The CVs show that the redox peaks disappear and the system shows electrical double layer capacitance only after 20 000 cycles. This is not surprising as molecular redox reactions typically exhibit poor stability on long-term potential cycling.47 The average capacitances obtained near the beginning of CD cycling (but after the loss of most of the pseudocapacitance component for AP-modified FLG) and near the end are listed in Table 2. The values reported are the averages from two samples of each type, and the uncertainties indicate the range of values. The data reveal that the 3FLGAP and 3FLGCP stacks have extraordinary stability. Although the stability of the 3FLGCP stack appears superior to that of the 3FLGAP stack, for the latter, the data in Figure 5E suggest that it is the loss of the pseudocapacitance contribution rather than double layer capacitance that accounts for this effect. While small differences in capacitance between samples may be accounted for by use of FLG prepared in different positions in the furnace and in different runs, the data strongly suggest that after 20 000 CD cycles, the capacitance for both types of modified stacks remains 3× higher than for the corresponding single sheet systems. This confirms that the ability of the grafted spacer groups to completely separate the FLG sheets and allow the full surface area of each FLG sheet to be accessible to electrolyte ions is maintained on long-term potential cycling. The discharge rate capabilities of single FLG sheets and 3 sheets stacks were also assessed from CV and CD data obtained after first recording 20 000 CD cycles at a current density of 10 μA cm−2. CVs were recorded at scan rates of 2−200 mV s−1 and the average discharge current density at 0.2 V was obtained from two replicate samples. Figure 6 shows plots of current

EIS data were used to further assess the performance of single sheet and 3-stacks of unmodified and modified FLG. This and all subsequent characterization of the materials was made on FLG samples modified with AP and CP groups through 7 h reaction in 20 mM diazonium salt solutions. The Nyquist plots obtained at 100 mV for FLG, FLGAP, FLGCP, 3FLGAP, and 3FLGCP are shown and briefly discussed in the Supporting Information (Figures S8 and S9). The plots for all samples are consistent with negligible interfacial charge transfer resistance as expected for purely electrical double layer capacitive behavior36−39 and fast ion diffusion to the FLG surfaces under the steady-state conditions of the EIS measurements.36,38,40−42 Figure 4 shows the Bode phase angle plots for the FLG materials. At 10 Hz the phase angle of approximately −85°

Figure 4. Bode phase angle plot for FLG, FLGAP, 3FLGAP, FLGCP, and 3FLGCP derived from EIS at 100 mV in 1 M HClO4.

confirms that all systems have almost ideal capacitive behavior.43 The frequency at −45° corresponds to the transition of the system from capacitive to resistive behavior.40,44 The relaxation time constant, τ0, can be calculated from this frequency (τ0 = 1/f 0), where τ0 is defined as the minimum time needed to discharge the capacitor which is operating at 50% of its maximum capacitance.44 τ0 is 1.2, 4.9, and 8.9 ms for FLG, FLGAP, and FLGCP respectively, indicating that the grafted layers have a small effect on ion movement to the FLG surface. Importantly, τ0 values do not increase for the 3-sheet stacks demonstrating that stacking does not hinder ion movement. These τ0 values are lower than often reported for other porous graphene and carbon materials (for example, 13.3 ms for self-stacked solvated graphene,5 17.8 ms for 3D porous rGO,38 26 ms for onion like carbon,45 and 700 ms for activated carbon45) and similar to that reported for vertically aligned and open-edge rGO electrodes (1 ms46). The electrochemical performance of modified FLG and modified FLG stacks was also evaluated by CV and galvanostatic charge−discharge (CD) measurements between 0 and 0.4 V. Figures 5A−C show CV responses prior to performing CD measurements. The rectangular shapes of the CVs of an unmodified single FLG sheet and a 3-sheet stack (Figure 5A) indicate fast double layer formation during charging and discharging.41,47 The area enclosed by the CVs, which is proportional to the capacitance, decreases for the 3sheet system as the material becomes HOPG-like. In contrast, 1425


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Figure 5. (A−C) CVs at 50 mV s−1 between 0 and 0.4 V in 1 M HClO4: (A) FLG and 3FLG; (B) FLGAP and 3FLGAP; (C) FLGCP and 3FLGCP. (D) CD between 0 and 0.4 V in 1 M HClO4 at 10 μA cm−2. (E) Cycle stability for CD testing at 10 μA cm−2 for FLG (blue), FLGAP (red) and 3FLGAP (green). (F) Cycle stability for CD testing at 10 μA cm−2 for FLGCP (orange) and 3FLGCP (purple).

Table 2. Average Capacitance Calculated from CD Testing of Two Samples of Each Type at 10 μA cm−2 for Cycles 500−2000 and Cycles 18 500−20 000 average capacitance/μF cm−2 sample FLG 3FLG FLGAP 3FLGAP FLGCP 3FLGCP

cycles 500−2000 16.8 10.3 15.3 56.8 15.2 51.5

± ± ± ± ± ±

0.4 1.0 0.9 9.3 0.6 1.9

cycles 18 500−20 000 17.2 10.1 13.0 43.7 13.9 52.9

± ± ± ± ± ±

0.4 1.0 0.1 1.5 0.6 2.8

density versus scan rate for one- and 3-sheet systems. All plots show two linear regions: from 0−10 mV s−1 and from 20−200 mV s−1 for the one-sheet samples, and from 0−20 mV s−1 and from 50−200 mV s−1 for the three-sheet samples. In both cases the slope of the plots in the faster scan rate regions show a small decrease compared with that at slower scan rates. At present we have no explanation for the two distinct regions, however the linearity of the plots for 3FLGAP and 3FLGCP stacks at higher scan rates, with no indication of a further decrease in rate capability, verifies fast diffusion of electrolyte ions, even at relatively high scan rate. Figures 7A and 7B show, respectively, the rate-dependent capacitance plots obtained from the CV data in Figure 6, and from CD measurements at charging rates of 6.25−50 μA cm−2. The differential capacitance vs discharge current density plots (Figure 7A) obtained from the data in Figure 6, show the expected two approximately linear regions for all samples: an initial sharp decrease in capacitance as the current density increases (this corresponds to the slow scan rate regions of Figure 6), followed by a plateau at higher current densities (corresponding to higher scan rates and shorter time scales). The CD measurements have a shorter time scale than the

Figure 6. Plots of scan rate vs discharge current at 0.2 V for (A) single FLG sheets, and (B) 3-sheet stacks. CVs were obtained after each sample had undergone 20 000 CD cycles at 10 μA cm−2. Each point represents the average of two samples, and the error bars show the range of values.

fastest CV measurements (see for example Figure 5D) and correspondingly, the rate capability plots of Figure 7B show almost constant integral capacitance vs discharge current density for each type of sample. Although the three-sheet systems show an initial small decrease in rate capability as the 1426


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20 000 CD cycles. There is no evidence that the discharge rate capability is decreased in the 3-sheet stacks compared with the corresponding single sheets of modified or unmodified FLG. Achieving graphene sheet separation with no decrease in the accessible surface area is a critical step in the development of real world devices from graphene. Although only supercapacitance was considered in this work, our findings also point to a general method for maximizing the accessible surface area in 3D graphene architectures for other applications that require a high accessible surface area, such as Li-ion batteries, gas adsorption and catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00280. Cartoon of a generalized multilayer film structure grafted from aryldiazonium salts, Scheme showing reduction of nitrophenyl groups; UV vis spectrum of FLG; IR spectra of CP- and NP-modified FLG; AFM images of unmodified and CP-modified FLG; CVs of FLG, FLGAP and FLGCP before and after EIS in 1 M HClO4; Nyquist plots of single FLG sheets and 3-sheet stacks; CVs in 1 M HClO4 of FLGAP and 3FLGAP before and after 20 000 CD cycles at 10 μA cm−2 (PDF)

Figure 7. Rate-dependent capacitance plots obtained from (A) the CV data in Figure 6 and (B) CD measurements. The data are the averages of two samples of each type and error bars show the range of values. The CD curves were obtained after each sample had undergone 20 000 CD cycles at 10 μA cm−2.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

current density increases, the decrease is only ∼15% over the range that current density increases 5 × (10−50 μA cm−2). Hence taken together, CV and CD measurements indicate that for all sample types (unmodified and modified single sheets and three-sheet stacks), there are relatively fast processes that contribute to the capacitance when operating at relatively short (and also presumably long) time scales, and slow processes that only contribute at long time scales. We have not investigated the origins of these processes however it is clear that the capacitance vs current density (and time scale) behavior of the modified three-sheet stacks show the same trends as for the corresponding single modified FLG sheets suggesting no significant additional barriers to ion diffusion are introduced by stacking the modified FLG sheets. Importantly, the fast processes lead to constant capacitance for 3FLGAP and 3FLGCP over the faster time scales relevant to supercapacitor devices.

ORCID

Paula A. Brooksby: 0000-0002-8494-7482 Alison J. Downard: 0000-0001-7661-6053 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Royal Society of New Zealand Marsden Fund (13-UOC-076). A.K.F thanks the University of Canterbury for her doctoral scholarship.

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CONCLUSION For the first time we have demonstrated that layered FLG structures can be fabricated using spacer groups that do not affect the capacitive performance of individual sheets, and that fully separate each stacked sheet giving capacitance that scales with layer number. CVD-grown FLG modified on one side with a sub-10 nm molecular layer grafted from aryldiazonium salts is porous to electrolyte solution and exhibits fast ion diffusion and full double layer formation at both FLG-solution interfaces. This porosity allows the sheets of chemically modified FLG to be stacked parallel to the underlying substrate while retaining full accessibility to electrolyte solution. We have demonstrated this behavior in acidic conditions using FLG modified with both AP and CP groups, suggesting the charge on the spacer layer is not important. The stacks of modified FLG have excellent stability with no loss of double-layer capacitance after 1427

ABBREVIATIONS AP, aminophenyl BET, Brunauer−Emmett−Teller CBD, carboxybenzene diazonium tetrafluoroborate CD, charge−discharge CNT, carbon nanotube CP, carboxyphenyl CV, cyclic voltammagram CVD, chemical vapor deposition EIS, electrochemical impedance spectroscopy ErGO, electrochemically reduced graphene oxide FLG, few layer graphene FLGAP, aminophenyl modified graphene FLGCP, carboxyphenyl modified graphene GO, graphene oxide HOPG, highly ordered pyrolytic graphite LBL, layer-by-layer NBD, nitrobenzenediazonium tetrafluoroborate NP, nitrophenyl rGO, reduced graphene oxide DOI: 10.1021/acsanm.8b00280 ACS Appl. Nano Mater. 2018, 1, 1420−1429

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SCE, saturated calomel electrode.

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Controlled Spacing of Few-Layer Graphene Sheets Using Molecular

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