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Tuning Mechanical, Electrical and Optical Properties of Flexible and Free-Standing Functionalized Graphene Oxide Papers Having Different Interlayer d-Spacing Uriel Marquez-Lamas, Eduardo Martinez-Guerra, Alberto Toxqui-Teran, Francisco Servando Aguirre Tostado, Tania E. Lara-Ceniceros, and JOSE BONILLA-CRUZ J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09961 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Tuning Mechanical, Electrical and Optical Properties of Flexible and Free-Standing Functionalized Graphene Oxide Papers Having Different Interlayer d-spacing. Uriel Márquez-Lamas,§,† Eduardo Martínez-Guerra, §,† Alberto Toxqui-Terán, § F. Servando Aguirre-Tostado, § Tania E. Lara-Ceniceros, §,† and José Bonilla-Cruz§,†,* †

Advanced Functional Materials & Nanotechnology Group, §Centro de Investigación en

Materiales Avanzados S. C. (CIMAV-Unidad Monterrey), Av. Alianza Norte 202, Autopista Monterrey-Aeropuerto Km 10, PIIT, Apodaca-Nuevo León, México, C.P. 66628. Email: [email protected], Tel: +52 81 1156 0809

1 ACS Paragon Plus Environment


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ABSTRACT Functionalized graphene oxide papers are assembled materials with a wide range of potential applications; however, scientific studies about how the physical properties of functionalized graphene oxide papers are affected by the presence of new functional groups, have not been addressed. Here, electrical resistance, optical band gap and mechanical properties of flexible and free-standing functionalized graphene oxide papers with nitroxide moieties (NGOP’s) have been studied and two of them (mechanicals and electricals) were correlated with the interlayer dspacing for the first time; wherein, a new insight about the key role that an optimum amount of functionalization agent plays over the physical properties of NGOP’s, is presented. In all cases, (functionalized or pristine) graphene oxide papers were prepared using anhydrous DMF instead water, in order to study and highlight the pure effect of the functionalized groups over their physical properties. The functionalized nanomaterials exhibited significantly improved physical properties, which can be tuned by varying the interlayer d-spacing; and this interlayer distance was systematically modified controlling the amount of functionalizing agent used. Thus, controlling the amount of functionalizing agent in the functionalization process is possible to obtain new assembled materials with modulated properties.


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INTRODUCTION. Since its discovery, graphene has been studied and synthesized by a lot of scientist around the world due to their remarkable range of potential applications. This bi-dimensional (2D) material has interesting physical properties;1-2 however, its zero valued band gap restricts its use in nanoelectronic devices;3 in spite of that, some techniques can be used to introduce a gap in graphene sheets, e.g. nanopatterning4-5 or chemical treatment6-8 which could have important processing advantages during a scaling up.9 On the other hand, among several methods used to obtain graphene, the chemical processing of natural graphite to obtain graphene oxide (GO) using strong acids10-11 seems to be the most promising method to produce a single or a few graphene layers at large-scale, although this methodology includes several chemical steps.12 Furthermore, GO has a wide range of oxygen-containing functional groups such as hydroxyl (OH) and epoxy groups (-C-O-C-) on the basal plane,13 and carboxyl groups (-COOH) at the edges.14 These functional groups open up the band gap, producing the possibility to modulate the electronic, optical, and mechanical properties of the material;15-22 and also, can be chemically modified with organic molecules to avoid their re-stacking and thus, to obtain well-dispersed functionalized GO into an organic matrix.23 Nonetheless, the careful choice of the organic molecule as well as their optimal concentration is an issue in lesser extend studied, which is crucial to obtain advanced materials with remarkable physical24-25 and chemical properties.26 On the other hand, graphene oxide paper (GOP) is a self-supporting thin film consisting of aligned GO layers with potential applications such as: gas and liquid-separation membranes27-28 anode material for lithium-ion batteries,29 among others. GOP can be prepared by pressurized ultrafiltration,30 air/liquid interface self-assembly31 or following the strategy reported in the pioneering work of Ruoff et al.,32 using vacuum filtration. Particularly, several studies have been



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focused to obtain GOP with improved mechanical properties (stiffness and strength, from aqueous dispersions) as well as an excellent electrical conductivity following two strategies: 1) through the intercalation of organic compounds33-34 or polymers35-36 between GO layers and, 2) chemically modifying –OH and –COOH groups with organic molecules, divalent ions37 or amines derivatives.28,

33, 38-40

To the best of our knowledge, physical properties (mechanical,

optical and electrical) of functionalized GOP have not yet been studied nor have been correlated with their interlayer d-spacing. Indeed, GO functionalization seems to be the best way to produce GO papers with improved properties; however, to reach this goal is necessary to understand the functionalization/structure relationship in order to know how this relationship can get to produce enhanced materials, nevertheless it is not trivial. On the other hand, scientific studies about how the density of functional groups affects the macroscopic properties have not been addressed, as well as some fundamental questions have not been answered yet, such as: Which is the optimal concentration of organic groups chemically bonded to GO layers that leads to obtain functionalized GO papers with improved properties? How is tuned the interlayer d-spacing in functionalized GO papers? and How this affects to their electrical, optical and mechanical properties?. On this basis, our main goal is to understand and correlate the macroscopic properties (electrical resistance and mechanical properties) with the interlayer d-spacing of NGOP’s for different functionalization levels (nitroxide group density). In this sense, in the present work the mechanical properties and electrical resistance of functionalized GO papers with nitroxide moieties (NGOP’s: NGOP1, NGOP2 and NGOP3) have been studied and correlated with the interlayer d-spacing for the first time. Functionalized GO papers possessing different functionalization levels and interlayer d-spacing (9.4, 10.1 and 11.2


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Å) were obtained by functionalizing the oxygen-containing functional groups (–OH and –COOH) in GO with nitroxide moieties using oxoammonium salts (OS: 0.63, 0.84 and 1.05 mmol). Herein we find that the mechanical properties and electrical resistance of NGOP’s can be modulated by varying the interlayer d-spacing; and this interlayer distance can be modified controlling the amount of functionalizing agent used. Further, optical band gap also was modulated as function of the functionalizing agent used, but was not correlated with the interlayer d-spacing because was obtained by UV-vis from aqueous dispersions. Thus, we demonstrate that the GO functionalization with a judicious amount of organic molecules, is the best way to obtain new assembled materials with versatile macroscopic properties.

EXPERIMENTAL SECTION Materials.

Graphene

oxide

(layers,

CxHyOz,

from

Sigma-Aldrich),

2,2,6,6-

tetramethylpiperidine-1-N-oxyl (TEMPO, C9H18NO, M.W. = 156.25 g/mol, + 99 %), N,N,Ntriethylamine (Et3N, M.W. = 101.19 g/mol, 99.5%), bromine (Br2), carbon tetrachloride (CCl4), dimethylformamide (DMF) and dichloromethane (CH2Cl2), were purchased from Sigma–Aldrich and were used without further purification. Methanol (MeOH) was acquired from CTR. Instrumentation. The interlayer d-spacing of each sample was obtained by power X-ray diffraction (XRD, Panalytical Empyream), 5° ≤ 2θ ≤ 80°, 10.16 s, 0.0167113 s, using Bragg– Brentano geometry. Chemical states and composition onto the sample surface was carried out by means of XPS on a Thermo Scientific Escalab 250Xi instrument. The base pressure during analysis was ∼10−10 mbar and the photoelectrons were generated with the AlKα (1486.68 eV) X-ray source using monochromator and with a spot size of 650 µm. The X-ray voltage and power were 14 kV and 350 W respectively. The acquisition conditions for the high-resolution 5 ACS Paragon Plus Environment


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selected region spectra were 20 eV pass energy, 45º take-off angle and 0.1 eV/step. Selected region spectra were recorded covering the C1s, O1s and N1s photoelectron peaks. The experimental error of the binding energy shift was smaller than this value. The recorded photoelectrons peaks were curve fitted using the Advantage Software V 5.41. Attenuated total reflectance (ATR) spectra were recorded on a Frontier MIR PerkinElmer ATR-FTIR spectrometer of 4000–400 cm-1 using 12 scans and 0.4 cm-1 of resolution at room temperature. Scanning electron microscopy (SEM) using an FEI Nova NanoSEM 200 was used to investigate the morphology of NGOP’s. Static mechanical uniaxial in-plane tensile test of GOP and NGOP’s were investigated using a dynamic mechanical analyzer (DMA Q800, TA Instruments, New Castle, DE). For this test, samples of GOP and NGOP’s were cut in strips of 5.3 mm of width by 10-12 mm in length with 50 µm of thickness, and were mounted using film tension clamps with a compliance of ca. 0.2 µm/N. The sample width was measured using a standard electronic calliper (Fowler High Precision, Inc., Newton, MA). The length between the clamps was measured by the DMA instrument, and the thickness was obtained using a standard digital micrometer (Mitutoyo, Co., Japan). All tensile tests were carried out at 28°C, in controlled-force mode with a preload of 0.01 N and force was loaded with a force ramp rate of 0.2 N/min and 2.0 N/min. For the optical characterization, a VARIAN Cary 5000 UV–Vis–NIR spectrophotometer with a resolution of 1 nm was used. The spectra were plotted in the wavelength range from 200 to 800 nm and were recorded for equal experimental conditions using 0.05 mg/mL in all cases. Band gaps were calculated with the aid of Tauc plot analysis from UV–Vis absorption spectra by assuming direct band gap, as described elsewhere.41-42 The electrical characterization of graphene oxide papers was measured using a Keithley-4200 semiconductor characterization system (SCS) using the standard two-point contact method. Silver top electrodes were 6 ACS Paragon Plus Environment

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evaporated by RF Sputtering onto graphene oxide papers using a shadow mask with a particular design to get different areas for top contacts. A probe station suitable for low noise measurements with micro-positioners and gold tips were used to reach the top contacts using the two-wire configuration. To get the reliable electrical resistance data, the two-point probe system was carefully placed on the functionalized GO papers, and three different sites of each sample were chosen to measure. Synthesis of oxoammonium salt (OS). 1.64 mL (0.032 mol) of bromine was added to a solution of 5 g in 100 mL of CCl4 of 2,2,6,6-tetramethylpiperidine-1-N-oxyl (0.032 mol). A brown solid was formed instantaneously, isolated and purified. OS obtained was dried over vacuum at room temperature overnight (yield = 98%). 1H NMR (CDCl3): δ(ppm): 2.3–2.7 (m, 6H), 1.7–2.0 (s, methyl, 12H). Graphene oxide papers functionalized with nitroxide groups (NGOP’s). Herein, three kinds of GOP functionalized with nitroxide groups (NGOP1, NGOP2 and NGOP3) were produced. To obtain NGOP1, 50 mg of GO (Aldrich) and 8 mL of DMF were placed into the glass reactor equipped with a cooling jacket, a condenser and a magnetic stirrer (DMF was used to improve GO dispersion). Then, the mixture was dispersed using an ultrasonic dismembrator during 30 min (750 W, amplitude = 10). After that, 221 µL (1.58 mmol) of triethylamine (Et3N) was added under vigorously stirred. Finally, a solution of 150 mg (0.635 mmol) of OS in 2 mL of DMF was added to the dispersion dropwise. The reaction was carried out at 2 °C under a N2 atmosphere during 4 h. The reaction product was exhaustively washed using clean methanol and was filtered through simple vacuum-filtration using a membrane filter Whatman of 0.2 µm pore size and 47 mm of diameter. After filtration, free-standing functionalized GO paper with nitroxide moieties (NGOP1) was obtained. Finally, was fully air-dried and was peeled-off from 7 ACS Paragon Plus Environment


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the membrane filter. To obtain NGOP2 and NGOP3 was followed the same experimental procedure described above and the amounts used are summarized in Table 1. Also a freestanding GO paper (without functionalization, GOP) was prepared for comparisons. Finally, GOP and NGOP’s were stored without further treatment.

Table 1. Functionalization of GO with nitroxide groups using three OS concentrations at 2°C and 4h. TAG GOP NGOP1 NGOP2 NGOP3

GO (mg)

Et3N

OS

(µL)

(mmol)

(mg)

(mmol)

DMF (mL)

50 50 50 50

221 295 368

1.58 2.11 3.63

150 200 250

0.63 0.84 1.05

10 10 10 10

RESULTS AND DISCUSSION. Graphene oxide layers (Sigma-Aldrich) were functionalized with nitroxide groups using oxoammonium salts (OS) as was described in the experimental section. During functionalization reaction, protons in –OH and –COOH groups on the surface and edges of GO; respectively, can be first abstracted by Et3N, resulting in phenoxides and carboxylates43 as intermediates species which then react with the 2,2,6,6,tetramethylpiperidine-1-oxonium cation to produce alkoxyamines.24 Moreover, alcohols and carboxylic acids are well known to oxidize under free radical conditions, and nitroxide itself is a stable free radical, which leads to obtain GO with several functional groups and alkoxyamines species attached to it. Meanwhile, the bromine anion


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is capped with Et3NH+ (forming a white precipitate), indicating that the reaction has taken place, as reveals Figure 1.

Figure 1. Schematic representation to obtain nitroxide-functionalized GO papers (NGOP’s). Following the procedure showed in Figure 1, nitroxide-functionalized GO layers were filtered and washed to produce “membranes” or “papers” decorated with nitroxide groups (NGOP’s) by vacuum filtration. Scanning electron microscopy (SEM) in Figure 1A reveals well-packed layers corresponding to functionalized GO with nitroxide moieties, through almost the entire crosssection of the sample. Figure 1B shows a digital image of NGOP’s as-obtained by vacuum 9 ACS Paragon Plus Environment


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filtration under controlled conditions with a diameter of 5.5 cm and Figure 1C shows a digital image of a rectangular strip of the same material, which exhibits a high bendability as freestanding paper. Herein, we find that using optimal amounts of OS, the -OH groups decreased ~ 20 % as well as a highly-functionalized GOP with nitroxide groups was obtained (as it will be demonstrated later by XPS). Functionalized GO papers with nitroxide groups. ATR-FTIR and interlayer d-spacing analysis (XRD). Graphene oxide paper without any functionalization (GOP) and functionalized GO papers with nitroxide groups (NGOP1, NGOP2, and NGOP3) were qualitatively analyzed and their structures were evidenced by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) as shown in Figure 2. Figure 2A shows characteristic absorption peaks of GOP:12, 28, 33 Stretching vibration of carboxyl groups (νs, C=O) at 1721cm-1; typical broad

νs, O-H at 3300 cm-1; νs, C=C in aromatic ring from unoxidized sp2 bonds centered at 1620 cm-1. At 1060 cm-1 νs, C-O of alkoxy groups and νs, C-O-C in epoxy group at 1230 cm-1. New vibrations were observed in NGOP’s which were attributed to νs, -CH3, νs, -CH2- and bending vibrations (δ) of N–O from nitroxide chemically bonded at 2910, 2871 cm-1 as well as at 779 and 603 cm-1. On the other hand, control of the d-spacing in GOP is a subject of great relevance which has been studied to a lesser extent. Particularly, GOP typically offer low permeation/adsorption rates due the fact that the d-spacing is too tight; nonetheless, scientific works regarding the control of interlayer distance are highly desirable in order to obtain high performance membranes, sensors or improved catalytic surfaces. Based on the above, we controlled the interlayer d-spacing of GOP functionalized with nitroxide moieties. OS concentration systematically was increased and according to the functionalization reaction, more and more nitroxide species were covalently 10 ACS Paragon Plus Environment

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bonded to the layers of GO to strengthen the interlayer adhesion, but also perturbations onto the GO structure (e.g. waviness, see later) and steric effects were produced

A)

Transmission intensity (a.u.)

NGOP3

NGOP2

NGOP1 CH CH3 2

GOP

carboxyl C=O aromatic C-C

hydroxyl -OH

4000

3600

3200

2800

2400

2000

carboxy C-O

1600

epoxy C-O-C

1200

alkoxy C-O

800

400

-1

Wavenumber (cm )

B) 7.9°

8.8°

9.4° 9.7° GOP NGOP1 NGOP2 NGOP3

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

7

8

9

10

11

(9.13 Å) (9.40 Å) (10.1 Å) (11.2 Å)

12

2θ θ (Degree)



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Figure 2. A) ATR-FTIR spectra of GOP and NGOP’s. B) XRD spectra of GOP and NGOP’s showing the interlayer distances (d-spacing). The X-ray diffraction (XRD) patterns shown in Figure 2B suggest the intercalation of nitroxide groups into the gallery spaces, which leads to an increase in the interlayer d-spacing. Also, full XRD spectra can be consulted in Figure S1 (see Support information). In Figure 2B pristine GO, exhibits a characteristic main peak at 2θ = 9.7° (corresponding to an interlayer d-spacing of d002 = 9.13 Å according to Bragg’s Law). Thus, when a low concentration of OS was used (0.63 mmol) to obtain NGOP1, a slight shift in the main peak was observed up to reach 2θ = 9.4° (d002 = 9.4 Å). In this case, a very low amount of nitroxide was covalently bonded to GO (as will be demonstrated by XPS), but was enough to produce a notorious change in the interlayer dspacing. Interestingly, increasing the amount of OS up to 0.84 mmol (to obtain NGOP2) a shift in the Bragg angle up to 2θ = 8.8° (d002 = 10.1 Å) was observed. In this case the amount of nitroxide chemically bonded (mainly in the edges) was increasing. Finally, to obtain NGOP3 was used 1.05 mmol of OS wherein the maximum interlayer d-spacing was obtained at 2θ = 7.9° (11.2 Å). Thereby, the interlayer d-spacing of NGOP’s as well as their specific surface area can be modulated by varying the concentration of OS added during the functionalization reaction. Although in this work, membranes were not characterized by Brunauer-Emmett-Teller (BET), XRD and SEM images undeniably suggest an increase in the specific surface area. GOP, NGOP1, NGOP2, NGOP3. XPS analysis. The atomic percent of nitroxide chemically bonded to the GO layers was determined by means of X-ray photoelectron spectroscopy (XPS), as well as to elucidate the surface state in GOP and NGOP’s as shown in Figure 3. Figure 3A shows the typical C1s XPS peak corresponding to GOP, which can be fitted into four 12 ACS Paragon Plus Environment

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distributions with binding energy34, 37, 40, 44-46 at 284.5, 286.6, 288.4 and 289.6 eV, corresponding to carbon sp2 (C=C), epoxy/hydroxyls (C-O-C, C-OH), carbonyl (C=O) and carboxylates (OC=O). For NGOP1, NGOP2 and NGOP3, a new peak centered at 285.2 eV was observed and was attributed to C–N bond of the piperidine ring from nitroxide.

A) GO

XPS Intensity (a.u.)


B) NGOP1

C-O

C=C

C-O C=C

O-C=O C=O 290

289

288

C-N

C=O 287

286

285

284

283

282

290

289

288

C=C

C-O

288

285

284

283

282

283

282

100

0

C=C C-O

C-N

C=O 289

286

D) NGOP3



C) NGOP2

290

287

Binding Energy (eV)

Binding Energy (eV)

287

286

285

C-N

C=O 284

283

282

290

289

288

Binding Energy (eV)

287

286

285

284

Binding Energy (eV)

55 50

C=C C-O C=O O-C=O C-N

E)

45

F)

35 30 25 20 15

O1s C1s

40


Percent composition by C1s XPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NGOP3

N1s

NGOP2

NGOP1

GOP

10 5 0

GOP

NGOP1

NGOP2

NGOP3

800

700

600

Graphene papers

500

400

300

200

Binding Energy (eV)



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Figure 3. XPS elemental analyses of deconvoluted C1s region of: A) GOP, B) NGOP1, C) NGOP2 and D) NGOP3; E) Atomic percent composition of deconvoluted C1s XPS spectra of GO papers: F) Wide scan spectra of each material highlighting N1s region. These results confirm that the nitroxide groups are covalently attached onto the GO surface. Indeed, as the OS content increases (~ 0.84 mmol), more and more nitroxide species are covalently attached and intercalated to the GO layers, which in agreement with the XRD analysis increase the interlayer d-spacing. Thus, the new C-N bond was notoriously increased as reveals Figure 3B and Figure 3C. Nevertheless, when a high amount of OS is used (>> 0.84 mmol), the amount of nitroxide incorporated does not change significantly (see Figure 3D), since we argue that this excess of OS was enough to saturate all possible reactive groups in the surface of GO; nonetheless, this excess leads to obtain a more exfoliated and oxidized material (see XRD and XPS). Further, in the case of NGOP2, a reduction around 20.12 % of C-O groups and a transformation of C-C bonds from sp3 to sp2 (~10%) was observed as reveals Figure 3E. This can be explained as follow: as the OS quantity progressively increases, -OH and –COOH groups react with OS to incorporate nitroxide species at the edges producing new alkoxyamines species, which leads to reduction of such groups as well as an increase in the interlayer d-spacing. In Figure 3E, all percentages of the oxidized materials were combined, thus GOP, NGOP1, NGOP2 and NGOP3 have: 61.6 % oxidized carbon; 56.3 % oxidized carbon and 1.5 % of functionalized carbon (C-N); 40.4 % oxidized carbon and 11.8 % of C-N; 45.9 % of the oxidized carbon and 10.9 % of C-N. Also, Figure 3E reveals that NGOP2 is the best functionalized material obtained here, since possess a high functionalization level (11.8 % of nitroxide covalently bonded to GO layers). Consequently, the amount of OS used to obtain NGOP2 (0.84 mmol) represents an optimal reaction condition. Finally, Figure 3F shows a wide scan spectrum 14 ACS Paragon Plus Environment

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of each material wherein in all cases C1s and O1s peaks were observed; meanwhile a signal of N1s (N-O bond) was only observed in nitroxide-functionalized GO papers at 406 eV. According to results obtained by Choi et al.,47 revealed that doped epitaxial graphene with nitroxide groups (4-amino-TEMPO) exhibited a signal N1s by XPS of N-O bond centered at 406 eV. In the present study, the N-O bond for all GOP’s was observed (as revealed in Figure S2, see support information) by N1s XPS at the same binding energy as reported in the literature.47 Mechanical properties of GOP and nitroxide functionalized graphene oxide papers (NGOP1, NGOP2 and NGOP3). Uniaxial tensile measurements of GOP and nitroxidefunctionalized GO papers (NGOP1, NGOP2 and NGOP3) were carried out in order to study both the effect of chemical functionalization and the interlayer d-spacing over their mechanical properties. Typically, GO papers or their composites are fabricated from colloidal suspensions of GO sheets in water.17, 32, 48 Water molecules play a key role in the improving mechanical properties of GOP, since GO is hydrophilic and always some amount of water is present between layers. This intercalates water produce hydrogen-bonding interactions between adjacent layers with oxygen-containing functional groups; thus a network is formed between layers that strengthens the paper structure dramatically.49 Indeed under certain water concentrations, mechanical properties of GOP exhibit tensile moduli in the range of 6-42 GPa32, 49 remaining highly flexible and ductile. However, in our case all papers (GOP and NGOP’s) have been prepared using anhydrous DMF instead water, to avoid the water effect in the mechanical properties and thus highlight the pure functionalization effect with nitroxide groups. On this basis, Figure 4 comparatively shows 15 ACS Paragon Plus Environment


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the stress (σ) - strain (ε) curves of GOP, NGOP1, NGOP2 and NGOP3 at two force rates: 0.2 N/min and 2 N/min, wherein the stiffness behavior (slope in the stress-strain curve) is showed as function of both the OS content and the interlayer d-spacing.

Figure 4. Stress-strain curves of GOP and nitroxide-functionalized graphene oxide papers (NGOP1, NGOP2 and NGOP3) at two force rates. GOP in Figure 4A exhibits very low tensile moduli (0.56-0.91 GPa) and toughness (12-22 MPa) attributed to an effect of the solvent (DMF) used. Interestingly, functionalized materials (NGOP’s) are stronger and stiffer than unmodified GOP independently of the force rate used. Further, when the strain rate increases from 0.2 to 2.0 N/min the ductility decreases, meanwhile the modulus and tensile strength increase -for very stiffer materials the effect is relatively small-. Thus, in the case of GOP and NGOP1 which exhibit a low stiff behavior, a change in the strain rate produces a positive change in the σ - ε slope curve (Figure 4A and Figure 4B); however, stiffer materials (NGOP2 and NGOP3) only exhibited a slight change in the moduli when the strain rate was increased in one order of magnitude. Despite the fact that NGOP2 and NGOP3 are stiffer materials; NGOP2 stands out because it exhibits the best toughness and strength of all 16 ACS Paragon Plus Environment

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functionalized materials as reveals Table 2. Also, in good agreement with XPS analysis, NGOP2 possess a high functionalization level, best reduction of C-O groups and the best mechanical properties. Therefore, NGOP2 is the best functional material produced here. Table 2. Mechanical properties of pristine GOP and GO papers functionalized with nitroxide groups (NGOP’s).

TAG

OS:GO

Load

Eo

E1

E2

Toughness

Ultimate

Ultimate

(mg:mg)

(N/min)

(GPa)

(GPa)

(GPa)

(MPa)

Strength

strain

(MPa)

(%)

-

0.2 2.0

0.56 0.91

0.78 1.56

0.82 1.44

12.9 22.7

13.0 22.3

1.9 1.9

NGOP1

(3:1)

0.2

0.76

1.30

1.8

44.9

24.6

2.5

NGOP1

(3:1)

2.0

1.49

2.27

2.51

33.5

26.0

2.1

NGOP2

(4:1)

0.2

4.2

5.47

5.2

66.4

58.9

1.9

NGOP2

(4:1)

2.0

5.70

5.25

4.66

91.6

66.0

2.3

NGOP3

(5:1)

0.2

2.74

4.83

4.63

17.1

33.5

1.0

NGOP3

(5:1)

2.0

3.25

4.23

5.01

33.3

44.9

1.3

GOP GOP

On the other hand, three regimens of deformation:32,

36-38

a straightening region at the

beginning followed by a roughly “elastic region” or “linear region” (although is not perfectly elastic32), and finally the last region considered as a plastic deformation, have been observed in GOP and NGOP’s. In all cases, we identify each modulus: E0, E1 and E2, corresponding to: straightening, elastic and plastic deformation region, respectively. Thus, the moduli increase as the OS content or the interlayer d-spacing increases, up to reach a critical or optimal value; after this, the modulus value decreases, as reveals Table 2. During functionalization, some oxygencontaining functional groups are consumed by OS to produce functionalized GO with nitroxide species (~ 11.8 % by XPS), which enhance the strength between layers. 17 ACS Paragon Plus Environment


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Moreover, as additional information, we show a plot of ⁄ = cos () Vs ε in Figure 4 for the experiments carried out to 0.2 N/min wherein we confirm the presence of 3 regimes: straightening, linear region and plastic deformation. ω is the frequency of strain oscillation (ε) and t is the time. Interestingly, following the ⁄ Vs ε curve for NGOP2 and NGOP3 in Figure 4C and Figure 4D; respectively, is possible to observe an abrupt change (as a step) attributed to the yielding zone (extensive plastic deformation) in the ⁄ at a specific strain (breaking point) which was attributed to the yielding point. Thus, NGOP2 exhibited more ability to absorb energy and high ductility even at an extensive plastic deformation. Besides, although NGOP3 exhibited a linear response until rupture and similar moduli to NGOP2 (due to the presence of nitroxide groups), it has less ductility with low plastic deformation and low toughness (see Table 2) because is a more oxidized material, as was disclosed above. Finally, Figure 5 schematize the functionalized layers with nitroxide groups, showing the waviness formed between adjacent graphene layers, the interlayer d-spacing, as well as compares each structural proposal with cross-sectional scanning electron microscopy (SEM) images of GOP and NGOP’s. Figure 5A shows a schematic representation about the interactions between oxygen-containing functional groups (face-to-face and edge-to-edge interactions)50-51 present in pristine GOP (d-spacing = 9.13Å); meanwhile Figure 5E shows their corresponding SEM image, wherein a well-stacked layer structure with a very smooth waviness was obtained. Indeed, if the waviness is relatively smooth, two consecutive layers can slip on each other easily, once they have overcome the straightening region. Moreover, controlling the concentration used of OS we can tune the interlayer d-spacing and as consequence, the mechanical properties of NGOP’s, which were improved in comparison with GOP. At the light of our results, we propose that as OS amount is increased, the most amounts of ripples or waves 18 ACS Paragon Plus Environment

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(produced during functionalization process) acquire an important role to improve the gripping between layers, and thus, contribute to enhance the macroscopic mechanical properties.

Figure 5. 2D schemes of: A) oxygen-containing functional groups interaction in pristine GOP. B)-D) interlayer d-distance and waviness produced by nitroxide-functionalized graphene oxide 19 ACS Paragon Plus Environment


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papers (NGOP1, NGOP2 and NGOP3). E)-H) Cross-sectional scanning electron microscopy (SEM) images of pristine GOP and NGOP’s corresponding to schemes A) to D). Figure 5B corresponding to NGOP1 represents a slight increase in the interlayer d-spacing (9.4Å) due to: i) the presence of some nitroxide species chemically attached to GO (lower OS amount used) and ii) the presence of small ripples. SEM in Figure 5F confirms the scheme proposed in Figure 5B, in this case ripples are more noticeable, without presence of cavities. Toughness was increased nearly 3.5 times respect to GOP. The best mechanical properties obtained in this contribution were exhibited by the sample NGOP2 (see Table 2 and Figure 5C). In order to explain our findings, we propose two mechanisms that improve the mechanical properties: Chemical mechanism: the chemical affinity between organic groups (nitroxide species) improves the strength between layers. Physical mechanism: Figure 5C shows an increase in the interlayer d-spacing (10.1 Å) into the galleries, and SEM in Figure 5G reveals the strong presence of ripples aligned one after another in some zones, which could improve the gripping between layers and thus contribute to enhance the mechanical properties. On the other hand, if a large amount of OS is used, a functionalized material with nitroxide groups is obtained with a higher interlayer d-spacing (11.2 Å); however, this material has some big cavities between stacks of layers as reveals Figure 5D and Figure 5H, which produces a stiff material with low toughness. Thus, a material possessing a high interlayer d-spacing (using high amounts of functionalizing agent) not necessarily will exhibit the best mechanical properties.

Electrical properties of GOP papers and nitroxide-functionalized GO papers (NGOP1, NGOP2, NGOP3). Electrical resistance values were obtained via in-plane measurements as reveals Figure 6A. Although transversal measurement is the best way to analyze the effect of d20 ACS Paragon Plus Environment

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spacing, the porous nature and low thickness of the samples promoted short circuit on I-V measurements which limits this kind of analysis. For in-plane measurements, a current is conducted partially through the bulk of the papers and over the stacked layer structure that includes some interlayer d-distances.

Figure 6. A) Graphene oxide paper with silver top electrodes for electrical measurements with 4200 semiconductor characterization system (SCS); B) I-V curves of nitroxide-functionalized graphene oxide papers for pristine GOP and NGOP1. C) I-V curves of nitroxide-functionalized graphene oxide papers for NGOP2 and NGOP3; D) Electrical resistance curves of all nitroxidefunctionalized graphene oxide papers (GOP, NGOP1, NGOP2 and NGOP3).



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GO samples are suggested to contain large sp2 domain sizes that are interrupted by sp3 bonds due to the presence of C-OH and C=O groups, showing thus an electrical resistance of 3.2 MΩ. Nitroxide groups produce NGOP's electrically inactive (increasing its electrical resistance). This effect is strongly related to the increase of the interlayer d-spacing as shown in Figure 6B and Figure 6C. It is believed that nitroxide groups blocks the electrical conduction in plane and also through the stacked layer structure. The inclusion of nitroxide functional groups limits the electrical connectivity among the graphitic domains by formation of new sp2 clusters, thus electrical resistance values increase from 3.22 x 106 Ω for GOP up to 2.05 x 108 Ω for NGOP3 with the higher OS content. The progressive increase of electrical resistance with the OS content is compiled in Figure 6D. In general, we explain the increase of the electrical resistance of the papers in terms of an effective destruction of the sp2 carbon network ensured by the functionalization with nitroxide groups, resulting in a decrease of the charge carrier transport in individual graphene sheets, which impede the electronic mobility. As the material is progressively modified with OS content, interactions among the clusters decrease. Additionally, presence of residual oxygen significantly hampers the carrier transport among the graphitic domains dominated by hopping or tunneling amongst the sp2 clusters.52 The improved increase of interlayer d-spacing due to the new attached species through the layers emerges here, thus, as an important methodology to modulate electrical resistance of GO papers.

Optical properties of GOP and nitroxide-functionalized GO papers (NGOP1, NGOP2, NGOP3). Ultraviolet–visible (UV–vis) spectroscopy measurements were employed to monitor the degree of functionalization of the GO papers. In this case the resulting optical band gap was not correlated with the interlayer d-spacing because the GOP and NGOP’s were well dispersed in


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deionized water (in all cases a concentration of 0.05 mg/mL was used) to obtain the UV-vis spectra. Nonetheless, this complementary study of characterization suggests a correlation between the absorption peak of each substrate and the amount of functionalized agent used during synthesis. UV-vis spectrum of GO exhibits two characteristic features that can be used as a means of identification:53 λmax ~225 nm corresponding to π→ π* transitions of aromatic C-C bonds (sp2 domains) and a shoulder at ∼300 nm which is attributed to n→ π* transitions of carbonyl and carboxyl groups. In the present work, the aqueous dispersions of GOP, NGOP1 and NGOP2 exhibited a λmax = 227.4 nm as shown in Figure 7A.

1x104

GOP NGOP1 NGOP2 NGOP3

(αhν ν)

1.5

2 (eV/cm)2

8x103

A)

2.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.5

GOP NGOP1 NGOP2 NGOP3

6x103

4x103

2x103

B)

0.0 0

200

400

600

800

0

Wavelenght (nm)

2

3

4

Photon energy (eV)

Figure 7. A) UV–Vis absorption spectra for GOP, NGOP1, NGOP2 and NGOP3 samples; B) Tauc plot using UV-Vis absorption spectra for prepared GO papers. The unchanged value of λmax indicates that aromatic rings are not in extended conjugation (π→ π* transitions); meanwhile an absorption peak was not completely defined for NGOP3 due 23 ACS Paragon Plus Environment


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to the fact that p-conjugation system was gradually decreased with increasing the functionalization degree. This optical behavior is another experimental evidence of functionalization with the nitroxide groups and at the same time, a limited absorption due to degree of exfoliation enhanced by the functionalizing organic groups. The optical band gap was calculated from Tauc plots considering a direct band gap as shown in Figure 7B. Although there is no general consensus about the band gap type for GOP, the Tauc plot for indirect band gap type was not well fitted. Figure 7B shows Tauc plot (i.e. (αhν)2 Vs hν) to determine band gap of different GO papers. The absorption coefficient (α) varies as a function of photon energy (hν) as αhν ∝ (hν-Eg)1/2 where Eg is the band gap. This means that direct band gap transition exists in GOP papers ad can be determined by drawing a tangent to the linear region of curve and its value is obtained by the intercept of the tangent on energy axis. Optical band gaps calculated from Tauc plots considering a direct band gap are 3.32, 3.54, 3.01 and 3.06 eV corresponding to GOP, NGOP1, NGOP2 and NGOP3, respectively. All these values are larger if they are compared with other reports.41-42 This fact could be related to the high functionalization level. The relatively high band gap of NGOP’s papers in comparison to graphene is also understood in terms of the presence of small sp3 fraction, as determined by XPS studies. The calculated band gap indicates a non-continuous decrease down to 3.01 eV for NGOP2 sample related with the efficiency of chemical method to incorporate organic groups between GO layers.

CONCLUSIONS. Nitroxide groups can produce different functionalization levels and interlayer d-spacing, which can to tune the macroscopic properties: optical, electronic and mechanical properties. Using an 24 ACS Paragon Plus Environment

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optimal amount of functionalizing agent (NGOP2, d-spacing = 10.1 Å), strong presence of ripples aligned one after another in some zones were observed, which improve the gripping between layers and thus contribute to enhance the mechanical properties. At large OS content (dspacing = 11.2 Å) very high amount of ripples and big cavities were observed, in detriment of the mechanical properties. Thus, to know the key role that a judicious amount of functionalizing agent plays in the functionalization is so really important in the finding of new materials with enhanced macroscopic properties. Electrical resistance values increased from 3.22 x 106 Ω for GOP up to 2.05 x 108 Ω for NGOP3 with the higher OS content. Finally, optical band gap also was modulated as function of the functionalizing agent used in aqueous dispersions of functionalized GOP with nitroxide moieties, wherein the maximum of optical absorbance decreased with the increase of OS content. Herein, we have found that the GO functionalization with small organic molecules in order to produce functionalized GO papers is an optional way to obtain new assembled materials with versatile macroscopic properties.

AUTHOR INFORMATION Corresponding Author Dr. José Bonilla-Cruz *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.



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Supporting Information Full XRD spectra of GOP and NGOP’s showing the interlayer distances (d-spacing). N1s XPS region corresponding to GOP, and functionalized GOP with nitroxide groups (NGOP’s). New NO bond was observed at 406 eV. ACKNOWLEDGMENT U. M.-L. and J. B.-C. give to thanks to Consejo Nacional de Ciencia y Tecnología (CONACyT)México, for the granting of a generous doctoral scholarship for U. M.-L. and for grant 1826312012 supporting this research. Also, authors thank to Nayeli Pineda for acquisition of SEM images and M.C. Gerardo Silva for XPS acquisitions. REFERENCES (1) (2) (3)

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SYNOPSIS TOC 1.0 N

GO

+

O

0.8 0.6

Oxoammonium salt (mmol)

NGOP2


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